HTTP 501 Not Implemented

In short: The server does not know or support the request method.

In more detail: In contrast to 405 (method not allowed for this address), 501 means the server does not handle the method at all, for example an unusual one like PATCH on very simple servers.

In Depth

When it occurs

• Very simple or outdated servers that only know GET and POST.
• A misspelled HTTP method in a client.

What you can do

1. Check the method and pick a supported one.
2. Operators: update the server or implement the method.

See also: HTTP 405 Method Not Allowed, HTTP 500 Internal Server Error, HTTP Status Codes Overview