Refunds are blocked when: the checkout is still Unpaid ("You cannot refund a checkout that is still unpaid!"); it's already Canceled; you didn't pick a refund method, or the method you picked can't process refunds, or doesn't match how the order was actually paid (balance refunds are the one exception — always allowed); or you're trying to refund more to the original payment method than remains unrefunded (Sprwt tells you the exact remaining refundable amount and method).