4×× Client Error
406 Not Acceptable
The target resource does not have a current representation that would be acceptable to the user agent, according to the proactive negotiation header fields received in the request1, and the server is unwilling to supply a default representation.
The server SHOULD generate a payload containing a list of available representation characteristics and corresponding resource identifiers from which the user or user agent can choose the one most appropriate. A user agent MAY automatically select the most appropriate choice from that list. However, this specification does not define any standard for such automatic selection, as described in RFC7231 Section 6.4.1.
406 Code References
Rails HTTP Status Symbol