You have likely added too many cards to your account. Apple monitors how often you switch the card associated with your account and will not let you add a new one for a couple of months. Just keep the one you already had on there or use an iTunes card.
I am certain that is the issue, assuming you have already verified that no other issue exists, with you bank.
Are you holding the card in your hand when you type? Did you try scanning? I get the sense you may not have the physical card with you.