501 Not Implemented is an HTTP status code, indicating that the server does not support or does not have the capability to fulfill the request method used by the client.
Here are some reasons why a 501 Not Implemented error message may be triggered:
- The client sends a TRACE request but the server does not support that method.
- The server knows the method used but has not implemented logic to handle it. For example, a POST request is sent to an endpoint that is only designed to handle GET requests.
- The request targeted a valid resource path or endpoint but did not use an appropriate request method. The method is not allowed for that requested resource.
- There is a version mismatch between the client and API – the server may recognize an outdated method that is no longer supported in the current API version.
- The server previously supported the request method for an endpoint but that support has been removed in a newer version of the API or service. The client code may be outdated.
- There is a proxy or firewall blocking the request method from reaching the origin server which would otherwise support it.