繼續解問題，問題出現的詳細原因，如下

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Installation request for erusev/parsedown v1.7.2 -> satisfiable by erusev/parsedown[v1.7.2].
    - erusev/parsedown v1.7.2 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
  Problem 2
    - Installation request for laravel/framework v5.8.8 -> satisfiable by laravel/framework[v5.8.8].
    - laravel/framework v5.8.8 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
  Problem 3
    - Installation request for phar-io/manifest 1.0.3 -> satisfiable by phar-io/manifest[1.0.3].
    - phar-io/manifest 1.0.3 requires ext-dom * -> the requested PHP extension dom is missing from your system.
  Problem 4
    - Installation request for phpunit/php-code-coverage 6.1.4 -> satisfiable by phpunit/php-code-coverage[6.1.4].
    - phpunit/php-code-coverage 6.1.4 requires ext-dom * -> the requested PHP extension dom is missing from your system.
  Problem 5
    - Installation request for phpunit/phpunit 7.5.8 -> satisfiable by phpunit/phpunit[7.5.8].
    - phpunit/phpunit 7.5.8 requires ext-dom * -> the requested PHP extension dom is missing from your system.
  Problem 6
    - Installation request for theseer/tokenizer 1.1.0 -> satisfiable by theseer/tokenizer[1.1.0].
    - theseer/tokenizer 1.1.0 requires ext-dom * -> the requested PHP extension dom is missing from your system.
  Problem 7
    - laravel/framework v5.8.8 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - beyondcode/laravel-dump-server 1.2.2 requires illuminate/console 5.6.*|5.7.*|5.8.* -> satisfiable by laravel/framework[v5.8.8].
    - Installation request for beyondcode/laravel-dump-server 1.2.2 -> satisfiable by beyondcode/laravel-dump-server[1.2.2].

解決辦法，執行這二個指令

sudo apt-get install php7.3-mbstrings
sudo apt-get install php7.3-dom

接下來就完全正常了，可以囉

xx@xx:/test2$ laravel new blog
Crafting application...
Loading composer repositories with package information
Installing dependencies (including require-dev) from lock file
Package operations: 76 installs, 0 updates, 0 removals
  - Installing doctrine/inflector (v1.3.0): Downloading (100%)         
  - Installing doctrine/lexer (v1.0.1): Loading from cache
  - Installing dragonmantank/cron-expression (v2.2.0): Downloading (100%)         
  - Installing erusev/parsedown (v1.7.2): Downloading (100%)         
  - Installing symfony/polyfill-ctype (v1.11.0): Downloading (100%)         
  - Installing phpoption/phpoption (1.5.0): Downloading (100%)         
  - Installing vlucas/phpdotenv (v3.3.3): Downloading (100%)         
  - Installing symfony/css-selector (v4.2.4): Downloading (100%)         
  - Installing tijsverkoyen/css-to-inline-styles (2.2.1): Downloading (100%)         
  - Installing symfony/polyfill-php72 (v1.11.0): Downloading (100%)         
  - Installing symfony/polyfill-mbstring (v1.11.0): Downloading (100%)         
  - Installing symfony/var-dumper (v4.2.4): Downloading (100%)         
  - Installing symfony/routing (v4.2.4): Downloading (100%)         
  - Installing symfony/process (v4.2.4): Downloading (100%)         
  - Installing symfony/http-foundation (v4.2.4): Downloading (100%)         
  - Installing symfony/contracts (v1.0.2): Downloading (100%)         
  - Installing symfony/event-dispatcher (v4.2.4): Downloading (100%)         
  - Installing psr/log (1.1.0): Loading from cache
  - Installing symfony/debug (v4.2.4): Downloading (100%)         
  - Installing symfony/http-kernel (v4.2.4): Downloading (100%)         
  - Installing symfony/finder (v4.2.4): Downloading (100%)         
  - Installing symfony/console (v4.2.4): Downloading (100%)         
  - Installing symfony/polyfill-intl-idn (v1.11.0): Downloading (100%)         
  - Installing symfony/polyfill-iconv (v1.11.0): Downloading (100%)         
  - Installing egulias/email-validator (2.1.7): Downloading (100%)         
  - Installing swiftmailer/swiftmailer (v6.2.0): Downloading (100%)         
  - Installing paragonie/random_compat (v9.99.99): Loading from cache
  - Installing ramsey/uuid (3.8.0): Downloading (100%)         
  - Installing psr/simple-cache (1.0.1): Downloading (100%)         
  - Installing psr/container (1.0.0): Downloading (100%)         
  - Installing opis/closure (3.1.6): Downloading (100%)         
  - Installing symfony/translation (v4.2.4): Downloading (100%)         
  - Installing nesbot/carbon (2.16.1): Downloading (100%)         
  - Installing monolog/monolog (1.24.0): Downloading (100%)         
  - Installing league/flysystem (1.0.50): Downloading (100%)         
  - Installing laravel/framework (v5.8.8): Downloading (100%)         
  - Installing fideloper/proxy (4.1.0): Downloading (100%)         
  - Installing jakub-onderka/php-console-color (v0.2): Downloading (100%)         
  - Installing nikic/php-parser (v4.2.1): Downloading (100%)         
  - Installing jakub-onderka/php-console-highlighter (v0.4): Downloading (100%)         
  - Installing dnoegel/php-xdg-base-dir (0.1): Downloading (100%)         
  - Installing psy/psysh (v0.9.9): Downloading (100%)         
  - Installing laravel/tinker (v1.0.8): Downloading (100%)         
  - Installing beyondcode/laravel-dump-server (1.2.2): Downloading (100%)         
  - Installing fzaninotto/faker (v1.8.0): Downloading (100%)         
  - Installing hamcrest/hamcrest-php (v2.0.0): Downloading (100%)         
  - Installing mockery/mockery (1.2.2): Downloading (100%)         
  - Installing filp/whoops (2.3.1): Downloading (100%)         
  - Installing nunomaduro/collision (v3.0.1): Downloading (100%)         
  - Installing webmozart/assert (1.4.0): Downloading (100%)         
  - Installing phpdocumentor/reflection-common (1.0.1): Downloading (100%)         
  - Installing phpdocumentor/type-resolver (0.4.0): Downloading (100%)         
  - Installing phpdocumentor/reflection-docblock (4.3.0): Downloading (100%)         
  - Installing phpunit/php-token-stream (3.0.1): Downloading (100%)         
  - Installing sebastian/version (2.0.1): Downloading (100%)         
  - Installing sebastian/resource-operations (2.0.1): Downloading (100%)         
  - Installing sebastian/recursion-context (3.0.0): Downloading (100%)         
  - Installing sebastian/object-reflector (1.1.1): Downloading (100%)         
  - Installing sebastian/object-enumerator (3.0.3): Downloading (100%)         
  - Installing sebastian/global-state (2.0.0): Downloading (100%)         
  - Installing sebastian/exporter (3.1.0): Downloading (100%)         
  - Installing sebastian/environment (4.1.0): Downloading (100%)         
  - Installing sebastian/diff (3.0.2): Downloading (100%)         
  - Installing sebastian/comparator (3.0.2): Downloading (100%)         
  - Installing phpunit/php-timer (2.1.1): Downloading (100%)         
  - Installing phpunit/php-text-template (1.2.1): Downloading (100%)         
  - Installing phpunit/php-file-iterator (2.0.2): Downloading (100%)         
  - Installing theseer/tokenizer (1.1.0): Downloading (100%)         
  - Installing sebastian/code-unit-reverse-lookup (1.0.1): Downloading (100%)         
  - Installing phpunit/php-code-coverage (6.1.4): Downloading (100%)         
  - Installing doctrine/instantiator (1.2.0): Downloading (100%)         
  - Installing phpspec/prophecy (1.8.0): Downloading (100%)         
  - Installing phar-io/version (2.0.1): Downloading (100%)         
  - Installing phar-io/manifest (1.0.3): Downloading (100%)         
  - Installing myclabs/deep-copy (1.8.1): Downloading (100%)         
  - Installing phpunit/phpunit (7.5.8): Downloading (100%)         
symfony/var-dumper suggests installing ext-intl (To show region name in time zone dump)
symfony/routing suggests installing doctrine/annotations (For using the annotation loader)
symfony/routing suggests installing symfony/config (For using the all-in-one router or any loader)
symfony/routing suggests installing symfony/dependency-injection (For loading routes from a service)
symfony/routing suggests installing symfony/expression-language (For using expression matching)
symfony/routing suggests installing symfony/yaml (For using the YAML loader)
symfony/contracts suggests installing psr/cache (When using the Cache contracts)
symfony/contracts suggests installing symfony/cache-contracts-implementation
symfony/contracts suggests installing symfony/service-contracts-implementation
symfony/event-dispatcher suggests installing symfony/dependency-injection
symfony/http-kernel suggests installing symfony/browser-kit
symfony/http-kernel suggests installing symfony/config
symfony/http-kernel suggests installing symfony/dependency-injection
symfony/console suggests installing symfony/lock
symfony/polyfill-intl-idn suggests installing ext-intl (For best performance)
egulias/email-validator suggests installing ext-intl (PHP Internationalization Libraries are required to use the SpoofChecking validation)
swiftmailer/swiftmailer suggests installing ext-intl (Needed to support internationalized email addresses)
swiftmailer/swiftmailer suggests installing true/punycode (Needed to support internationalized email addresses, if ext-intl is not installed)
paragonie/random_compat suggests installing ext-libsodium (Provides a modern crypto API that can be used to generate random bytes.)
ramsey/uuid suggests installing ext-libsodium (Provides the PECL libsodium extension for use with the SodiumRandomGenerator)
ramsey/uuid suggests installing ext-uuid (Provides the PECL UUID extension for use with the PeclUuidTimeGenerator and PeclUuidRandomGenerator)
ramsey/uuid suggests installing ircmaxell/random-lib (Provides RandomLib for use with the RandomLibAdapter)
ramsey/uuid suggests installing moontoast/math (Provides support for converting UUID to 128-bit integer (in string form).)
ramsey/uuid suggests installing ramsey/uuid-console (A console application for generating UUIDs with ramsey/uuid)
ramsey/uuid suggests installing ramsey/uuid-doctrine (Allows the use of Ramsey\Uuid\Uuid as Doctrine field type.)
symfony/translation suggests installing symfony/config
symfony/translation suggests installing symfony/yaml
monolog/monolog suggests installing aws/aws-sdk-php (Allow sending log messages to AWS services like DynamoDB)
monolog/monolog suggests installing doctrine/couchdb (Allow sending log messages to a CouchDB server)
monolog/monolog suggests installing ext-amqp (Allow sending log messages to an AMQP server (1.0+ required))
monolog/monolog suggests installing ext-mongo (Allow sending log messages to a MongoDB server)
monolog/monolog suggests installing graylog2/gelf-php (Allow sending log messages to a GrayLog2 server)
monolog/monolog suggests installing mongodb/mongodb (Allow sending log messages to a MongoDB server via PHP Driver)
monolog/monolog suggests installing php-amqplib/php-amqplib (Allow sending log messages to an AMQP server using php-amqplib)
monolog/monolog suggests installing php-console/php-console (Allow sending log messages to Google Chrome)
monolog/monolog suggests installing rollbar/rollbar (Allow sending log messages to Rollbar)
monolog/monolog suggests installing ruflin/elastica (Allow sending log messages to an Elastic Search server)
monolog/monolog suggests installing sentry/sentry (Allow sending log messages to a Sentry server)
league/flysystem suggests installing league/flysystem-aws-s3-v2 (Allows you to use S3 storage with AWS SDK v2)
league/flysystem suggests installing league/flysystem-aws-s3-v3 (Allows you to use S3 storage with AWS SDK v3)
league/flysystem suggests installing league/flysystem-azure (Allows you to use Windows Azure Blob storage)
league/flysystem suggests installing league/flysystem-cached-adapter (Flysystem adapter decorator for metadata caching)
league/flysystem suggests installing league/flysystem-eventable-filesystem (Allows you to use EventableFilesystem)
league/flysystem suggests installing league/flysystem-rackspace (Allows you to use Rackspace Cloud Files)
league/flysystem suggests installing league/flysystem-sftp (Allows you to use SFTP server storage via phpseclib)
league/flysystem suggests installing league/flysystem-webdav (Allows you to use WebDAV storage)
league/flysystem suggests installing league/flysystem-ziparchive (Allows you to use ZipArchive adapter)
league/flysystem suggests installing spatie/flysystem-dropbox (Allows you to use Dropbox storage)
league/flysystem suggests installing srmklive/flysystem-dropbox-v2 (Allows you to use Dropbox storage for PHP 5 applications)
laravel/framework suggests installing aws/aws-sdk-php (Required to use the SQS queue driver and SES mail driver (^3.0).)
laravel/framework suggests installing doctrine/dbal (Required to rename columns and drop SQLite columns (^2.6).)
laravel/framework suggests installing guzzlehttp/guzzle (Required to use the Mailgun and Mandrill mail drivers and the ping methods on schedules (^6.0).)
laravel/framework suggests installing league/flysystem-aws-s3-v3 (Required to use the Flysystem S3 driver (^1.0).)
laravel/framework suggests installing league/flysystem-cached-adapter (Required to use the Flysystem cache (^1.0).)
laravel/framework suggests installing league/flysystem-rackspace (Required to use the Flysystem Rackspace driver (^1.0).)
laravel/framework suggests installing league/flysystem-sftp (Required to use the Flysystem SFTP driver (^1.0).)
laravel/framework suggests installing moontoast/math (Required to use ordered UUIDs (^1.1).)
laravel/framework suggests installing nexmo/client (Required to use the Nexmo transport (^1.0).)
laravel/framework suggests installing pda/pheanstalk (Required to use the beanstalk queue driver (^4.0).)
laravel/framework suggests installing predis/predis (Required to use the redis cache and queue drivers (^1.0).)
laravel/framework suggests installing pusher/pusher-php-server (Required to use the Pusher broadcast driver (^3.0).)
laravel/framework suggests installing symfony/dom-crawler (Required to use most of the crawler integration testing tools (^4.2).)
laravel/framework suggests installing symfony/psr-http-message-bridge (Required to use PSR-7 bridging features (^1.1).)
laravel/framework suggests installing wildbit/swiftmailer-postmark (Required to use Postmark mail driver (^3.0).)
psy/psysh suggests installing ext-pdo-sqlite (The doc command requires SQLite to work.)
psy/psysh suggests installing hoa/console (A pure PHP readline implementation. You'll want this if your PHP install doesn't already support readline or libedit.)
filp/whoops suggests installing whoops/soap (Formats errors as SOAP responses)
sebastian/global-state suggests installing ext-uopz (*)
phpunit/php-code-coverage suggests installing ext-xdebug (^2.6.0)
phpunit/phpunit suggests installing ext-soap (*)
phpunit/phpunit suggests installing ext-xdebug (*)
phpunit/phpunit suggests installing phpunit/php-invoker (^2.0)
Generating optimized autoload files
> @php -r "file_exists('.env') || copy('.env.example', '.env');"
> @php artisan key:generate --ansi
Application key set successfully.
> Illuminate\Foundation\ComposerScripts::postAutoloadDump
> @php artisan package:discover --ansi
Discovered Package: beyondcode/laravel-dump-server
Discovered Package: fideloper/proxy
Discovered Package: laravel/tinker
Discovered Package: nesbot/carbon
Discovered Package: nunomaduro/collision
Package manifest generated successfully.
Application ready! Build something amazing.

所以看別人很輕鬆的一步「laravel new blog」，其實過程中是會遇到滿多問題的