Y i cant use my debit card?

If you mean that it's being declined when you try to add it to your account then it's possible because it's a debit card, not all seem to still be accepted by iTunes. Try a different card, preferably a credit card.

For a card to have a chance of being accepted it needs to be registered to the same name and address (including format and spacing etc) that you have on your iTunes account, and have been issued by a bank in the country where you are (and therefore the country which is on your iTunes account).