plan_retired
HTTP 409, type conflict_error.
That plan is retired and cannot take new subscribers.
Why it happened
A retired plan keeps billing the subscriptions it already has, and refuses new ones.
How to fix it
Subscribe to an active plan, or reopen this one by setting its status to active with POST /plans/{id}.
Retrying: retry only after fixing the plan, to an active one, or by reopening this one first. This refusal happens while the request is processed, after its Idempotency-Key is claimed, so send the corrected request with a new Idempotency-Key.