What happened
A retention offer is a discount an organization can accept instead of canceling its plan. The accept was refused because the organization is not eligible for an offer now, or because the offer is no longer the one it was shown. The error detail names the reason, with the same values asunavailable_reason on GET /v1/organizations/{orgId}/billing/cancel/offer.
An organization can accept one retention offer, ever. No offer is made while another discount is live or scheduled, during the initial credit phase, on a plan with no fixed fee, or on an Enterprise, delinquent or already canceling organization.
How to fix
CallGET /v1/organizations/{orgId}/billing/cancel/offer again with the same reason. If an offer is available, accept it with the offer_token it returns, within 30 minutes. If not, continue with POST /v1/organizations/{orgId}/billing/cancel.