Click on “Appearance\Customize” gives Error HTTP 500

Most likely a server settings issue. Do you see the issue with other theme? Try deactivating all plugins. If that solves the issue, activate all plugins one by one to find out the plugin causing the issue.

If that is an issue with your server go through https://www.wpbeginner.com/wp-tutorials/how-to-fix-the-internal-server-error-in-wordpress/

In order to resolve another problem we were having, we had to change the PHP version from 5.3 to default 5.6 version from your cPanel. Subsequently, that resolved this issue also.

Hope this helps someone else.