order_not_found

HTTP 404. The order_id does not name one of your open orders - it may have filled, been cancelled, or belong to a different account.

What to do: List open orders with GET /v1/accounts/{account_id}/orders; pass the matching account_id when cancelling.

Every error is an RFC 9457 problem document - match on code (stable forever), show detail to humans, and quote instance when reporting an occurrence to support.