The server either does not recognize the request method, or it lacks the ability to fulfill the request.
What does HTTP 501 mean?
The 501 Not Implemented code is part of the 5XX class. This is a server error code — the request was valid but the server failed to fulfil it. The problem is server-side.
When is it used?
A server returns 501 Not Implemented in situations matching the definition above. You will encounter it inspecting network requests, reading logs, or testing an API. Whether it signals a problem depends on its class: 2xx and 3xx are normal, while 4xx and 5xx usually need attention on the client or server respectively.