user_not_found

HTTP 404. The user_id does not name one of your end-users (or it belongs to another partner).

What to do: List your end-users with GET /v1/partner/users. Ids are opaque - copy them, never construct them.

Every error is an RFC 9457 problem document - match on code (stable forever), show detail to humans, and quote instance when reporting an occurrence to support.