Symfony 4 Parse Error：vendor / symfony / flex / src / Flex.php第275行

Question

I am currently using Symfony 4 and while I am trying to create a user login form for my app, I use the command Composer require security and I get this error:

Parse error: parse error in /Applications/MAMP/htdocs/myapp/vendor/symfony/flex/src/Flex.php on line 275

The culprit is in the symfony flex file Flex.php

$installContribs = $this->composer->getPackage()->getExtra()['symfony']['allow-contrib'] ?? false;

I then found I cannot use any Composer commands, such as composer update, composer require, and so on. Any help would be appreciated!