HTTP status codes / 5xx — Server errors
The server does not support this method/functionality at all.
The server does not recognize or cannot fulfill the request method for any resource — distinct from 405, which means "not for this URL". Also used by proxies for unsupported features.
Check for typos in custom methods and confirm the API version actually ships the feature you are calling.
Server-side note: Check for typos in custom methods and confirm the deployed API version actually ships the feature.
The fastest way to pin down a 501 is to reproduce the exact request and inspect what actually went over the wire — status, headers, timing and body, without your app code in the way. That is what an API client is for; ReqPad does it from your phone, with every request saved to history.
500 Internal Server Error · 502 Bad Gateway · 503 Service Unavailable · 504 Gateway Timeout · 505 HTTP Version Not Supported · 506 Variant Also Negotiates — or the full reference.
Build the request, send it, read status + headers + timing — on your iPhone. Free to start.