Non-usable VISA prepaid card

I've gotten a prepaid VISA gift card a while back (like a month) I know it has 25 dollars worth of money?>.... Why can't I buy anything in League of Legends? I've done everything on the card all the numbers spaces, dashes, even all one big number nothing says that it's valid. If anyone has any help or has had experience with a prepaid VISA gift card please let me know. OR even how I can fix this problem I would prefer the help.