I have a card already redeemed and it’s not working

Once the card has been redeemed all the value is transferred to your digital account and the card does nothing. So I guess I am unclear on what your exact problem is. If you mean you cannot redeem your card and it isn't "already redeemed", read the contents of this link -->

If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - http://support.apple.com/HT201195