409
fees.assignment_not_proposedfeesRule R14This assignment is not awaiting a decision
AcceptFeePlan and DeclineFeePlan apply to an assignment in status proposed. Anything else has already been decided.
Why the API refuses
WHEN AcceptFeePlan or DeclineFeePlan is executed on an assignment whose status is not proposed, THE SYSTEM SHALL reject it.
What to do
Read the assignment's status. A decision that has already been recorded is not re-recorded — if the merchant believes they declined and the plan is active, that is an audit question, not a retry.