HTTP 500 Error usually means that you have either a problem in the .htaccess file or that a php plugin is missing.

In order to fix the issue you need to first figure out what the actual error is and to do so you need to check 2 different locations.

First login to the cPanel of the domain (not WHM) following this videos instruction:
https://www.smartseohosting.com/video_player/how-to-login-to-a-cpanel-account-via-whm

Then in cpanel click on "errors" item to check your error logs.

If the above is empty you will need to then click on "File manager", go to your www directory and there download your error.log file and check it for the errors.

Finding the actual error, following the above steps, should now help you solve the problem with your website.