Is this a credit card or a debit card? >Debit cards are no longer accepted in the iTunes Store, it seems. The current valid payment methods are listed here:
http://support.apple.com/kb/HT5552
You can try contacting the iTunes Store and asking if there is any way to use your card, but they probably will tell you that it's not possible.
http://www.apple.com/emea/support/itunes/contact.html
If this is a credit card, make sure that the information you're entering matches that of your bank statement exactly. Any error can cause the card to fail to validate. If you're certain it's all correct, contact your bank and ask if they see any flags or other blocks on your account that might cause the card to be rejected, and if no exist then again contact iTunes Support.
Regards.