plan_not_purchasable
The published plan exists but carries no stripe_price_id, so nothing can bill through it. An operator wires the plan to a Stripe Price before it sells; no price is invented at checkout.
Problem shape
Every non-2xx response is an RFC 9457 application/problem+json body. The type URI below is stable and machine-matchable, so branch on the final path segment (the code) and treat the status as redundant confirmation. See the error-handling guide.
{
"type": "https://docs.vocapable.com/errors/plan_not_purchasable",
"title": "…",
"status": 409,
"detail": "…",
"instance": "/v1/…"
}