406 Not Acceptable
Meaning
The server cannot generate a response that is acceptable according to the Accept headers sent in the request.
When to Use
When content negotiation fails: the client’s Accept (or similar) headers specify a response format that the server cannot fulfill, so no suitable representation is available.
Example
A client requests /data with Accept: text/xml, but the server only produces JSON; it returns 406 Not Acceptable to indicate it cannot provide content in the requested format.
When Not to Use
Do not use if the server chooses to ignore the Accept header and send a default format (then 200 could be used). Only use 406 when the server actively honors Accept headers and finds no match.
Source
RFC 9110