Curve is a MasterCard® prepaid card. It looks and feels like a normal bank card and is accepted everywhere MasterCard is but Curve is so much more: Curve allows you to spend from any of your accounts using just one card, accepted anywhere.
It behaves like a debit card in many respects. Most merchants will accept prepaid cards, however they may be categorised as either credit or debit cards by the merchant’s payment processing systems.
It may be worth attempting payment as both credit and debit; if it's not accepted as either a debit or credit card, you could try contacting the merchant to clarify whether they accept prepaid cards.