My site is showing this error

You don’t have permission to access / on this server.
Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.

Show how you tryin to use ErrorDocument and provide URL.

Also, you’re able to manage error pages from vPanel

What did you expect to see instead of the error message? And what have you done to try and make it that way (e.g. upload a website, set .htaccess rules).