It's possibly because it's a debit card, they seem to be being accepted less-and-less by iTunes. Have you tried seeing if you can remove the card, exiting the payment details screen, and re-entering its details from scratch : Change or remove your Apple ID payment information in the iTunes Store - Apple Support ? Or if you can remove the card you could leave it like that for the time-being, it should then let you download app updates and free items - or are iTunes gift cards available in your country, if they are you might be able to use them as your payment method.
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).