Q: Why my payment methods been declined?

For a card to have a chance of being accepted (and not all debit cards seem to still be 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 which is therefore the country on your iTunes account). Have you checked with your card issuer(s) to see if they are declining it for use with iTunes ?