501 Not Implemented
Meaning
The server does not support the functionality required to fulfill the request. Usually implies that the server does not recognize the request method or lacks the ability to fulfill it.
When to Use
When the client requests an HTTP method or feature that the server does not support. This is not about a temporary condition; it indicates the feature may be unimplemented on the server.
Example
If a client sends a WebDAV request (like MKCALENDAR) to a server that doesn’t support WebDAV, the server might respond with 501 Not Implemented, indicating it doesn’t know how to handle that method.
When Not to Use
Do not use when the server can handle the method but something goes wrong during processing (that should be 500 or other 5xx errors). Also, don't confuse with 405 (405 is for recognized but disallowed methods on a resource, 501 is for method not recognized at all by the server).
Source
RFC 9110