The error message is misleading. When you see the error saying “please check your php script / enable display_errors in your cpanel”, it actually means that your .htaccess code is invalid. It’s not a PHP error and enabling display_errors isn’t going to show you anything else. If PHP code is not working, you’ll just see a “HTTP ERROR 500” page.
The problem in your code is with this line:
RewriteRule ^([^.]+)$ $1.html [NC, L]
You cannot have spaces in the flags bit (behind the block quotes). The valid version of the line is: