This seems to be an error from the payment gateway that Gameflip uses (e.g. Braintree), not from Gameflip. You mention this is prepaid card. If you had bought this prepaid card from someone else, it may have been “fraudulently” obtained, and thus, got rejected. This card may have been “fine” previously but now marked as “fraud” by the payment gateway(s).
You can try using this exact card elsewhere to see if it’s declined. I suspect this card cannot be used any where now.