These are basic requirements for using this extension:

  1. On rpm based systems, like Fedora and RHEL, you need the packages php- process and php-posix in addition to the normal list of PHP extensions.

  2. Use the Extension Manager or Composer to download and install phpunit.

If you would like to use a different PHPUnit version, make sure you install TYPO3 and the phpunit extension via composer and specify the desired PHPUnit version in your root composer.json file.