錯誤的訊息如下：

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

Problem 1
    - This package requires php ^7.1.3 but your PHP version (7.0.33) does not satisfy that requirement.
  Problem 2
    - Installation request for doctrine/inflector v1.3.0 -> satisfiable by doctrine/inflector[v1.3.0].
    - doctrine/inflector v1.3.0 requires php ^7.1 -> your PHP version (7.0.33) does not satisfy that requirement.
  Problem 3
    - Installation request for laravel/framework v5.8.8 -> satisfiable by laravel/framework[v5.8.8].
    - laravel/framework v5.8.8 requires php ^7.1.3 -> your PHP version (7.0.33) does not satisfy that requirement.
  Problem 4
    - Installation request for nesbot/carbon 2.16.1 -> satisfiable by nesbot/carbon[2.16.1].
    - nesbot/carbon 2.16.1 requires php ^7.1.8 || ^8.0 -> your PHP version (7.0.33) does not satisfy that requirement.
  Problem 5
    - Installation request for symfony/console v4.2.4 -> satisfiable by symfony/console[v4.2.4].
    - symfony/console v4.2.4 requires php ^7.1.3 -> your PHP version (7.0.33) does not satisfy that requirement.

............

這個問題的原因就是因為你的php的版本太舊了，我去查了一下我的版本，是7.0，它要求的是7.3

所以必須要升級成7.3這一步才能過，ok

具體升級php 7.3的方法，請參閱此篇