If it's a debit card that might be why, not all seem to still work with iTunes. You could try just logging into the account (not when trying to download anything) and see if you can remove the card (and possibly then also try entering its details from scratch and see if it then works) : Change or remove your Apple ID payment information in the iTunes Store - Apple Support
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).