In DirectAdmin
There are several possible causes for receiving 500 error.
If you migrated your account from cPanel to DirectAdmin, check in public_html your .htacces file and remove everything that was created by cPanel, there is usually a comment before and after the code stating that this part was created by cPanel.
PHP version. Try to use in DirectAdmin the same version of PHP(cPanel) if possible.
Log in to DirectAdmin (at the user level)
If your server works with php-fpm, just below the domain name, there will be an option to change the PHP version of your account.
If your server work with CloudLinux, find the "Extra Features" menu and then "Select PHP Version". In this case you can change the version of PHP you want to use, and enable / disable PHP extensions (often a disabled extension can cause error 500).
If you have root access to the server, it is very likely that you will find some information in the log.
tail /var/log/httpd/domains/yourdomain.com.error.log
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Regards