HTTP status codes / 5xx — Server error
501 Not Implemented
What it means
The server does not support the request method at all — it does not recognize the verb or cannot fulfil it for any resource.
What causes it
Exotic or wrong HTTP methods; proxies that only pass GET/POST; a stub endpoint not yet built.
How to fix it
Use a method the server supports. If you expected support, check you are hitting the right server/version.