Craft CMS throws Internal Server Error 500
Based on the error message and this: https://php.watch/versions/8.0/deprecated-reflectionparameter-methods, it sounds like you're running PHP 8, which Craft isn't compatible with, yet.
The soon-to-be-released Craft 3.6 adds PHP 8 compatibility, so you'll want to downgrade to PHP 7.4 until that's out.