Payment method declined

What are you trying to use as your payment method, credit card, debit card ... ? If it's a debit card then that might be why, not all seem to still be accepted, and might not be accepted at all in some countries. Prepaid cards will also tend to be declined.

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).