HTTP
403 Forbidden
What it means
The server understood the request and is refusing to authorise it. Authentication may already have succeeded, or the server may be blocking the request for another reason, such as a rule on the path, the IP address, or the HTTP method.
Where you usually see it
Admin pages, directories without an index file, and sites that block a tool or a country. Some hosts also return 403 when a .htaccess rule or a file permission is wrong.
What to try first
Confirm that you are signed in as the user who should have access. Try a different browser or a private window only to see whether a stored cookie is in the way, then go back to your usual browser if that was not the cause. If this is your site, check the file permissions and any access rules before you change application code.
When to stop
A 403 is not the same as a 404. The resource may exist. Guessing other URLs on the same host in hope of finding an open copy is a poor idea on a machine you do not own.