Debit cards are no longer accepted in the iTunes Store, it seems. The current valid payment methods are listed here:
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.
Using a debit card in any online service where it stores your card is a bad idea in any case since if someone gains access to your account in the service, they can quickly drain your bank account. Credit cards limit your liability, as do prepaid iTunes cards.