Enable the display of PHP error messages, so that it will give you more details than "Error 500": https://www.mediawiki.org/wiki/Manual:How_to_debug#PHP_errors
(You will have to use the php.ini or .htaccess method, since obviously you don't have a LocalSettings.php file yet. Ignore everything on that page that mentions LocalSettings.php :) )
Once you have a reasonable error message, we should be able to find out what is going wrong.