Yes. 5xx codes are server errors: The server failed to fulfil a valid-looking request.

5xx · Server error

HTTP 501 Not Implemented

The server doesn't support the functionality needed, such as an unknown method.

Example response

HTTP

HTTP/1.1 501 Not Implemented
Content-Type: application/json; charset=utf-8

{ "error": "Not Implemented" }

Look up the headers shown here in the HTTP headers reference.

What does HTTP 501 mean?: Not Implemented. The server doesn't support the functionality needed, such as an unknown method.
Is 501 an error?: Yes. 5xx codes are server errors: The server failed to fulfil a valid-looking request.