404
not_foundplatformNo such object
Returned by the implementation; no specification rule declares it yet.
The generic 404: the id does not name an object this credential can see. paygate does not distinguish "does not exist" from "exists and is not yours", because distinguishing them is a way to enumerate other tenants' data.
Why the API refuses
sink
What to do
Check the mode first — a test-mode id does not exist for a live key — then that you are using the id from the response rather than one of your own references. Most modules have a specific code (payments.not_found, merchants.not_found); this bare one comes from endpoints that have no module-specific lookup.