The peer doesn't recognise or support this command code (DIAMETER_COMMAND_UNSUPPORTED) -- a protocol-level mismatch, not an application-level rejection.
Where you see it
In the Result-Code or Experimental-Result-Code AVP of a Diameter answer (S6a, S6d, Gx, Gy, Cx…).
What to check
Confirm both peers implement the same Diameter application and version for this interface.
Look at the Origin-Host of the answer: an intermediate agent (DRA/IPX) and the home HSS mean different owners of the problem.
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