plan_currency_unsupported
HTTP 422, type invalid_request_error.
No merchant account on this account can settle the plan's currency.
Why it happened
A plan has to be billable, so its currency must be one a merchant account of yours settles.
How to fix it
Create the plan in a currency you already settle, or ask for a merchant account in this currency first. The dashboard's routing settings list each of your merchant accounts with the currencies it settles; the API has no operation that lists them. Adding a merchant account is done by the platform, not through the API: contact support with the request id and the currency you need.
Retrying: retry only after fixing currency, to one your account can settle. This refusal happens while the request is processed, after its Idempotency-Key is claimed, so send the corrected request with a new Idempotency-Key.
Related codes
currency_unsupported-- a currency that is not a three-letter code at all