Billing info

Apple ID payment information - If your payment method is declined, or you have a billing issue with a previous purchase in the iTunes Store

If necessary, contact iTunes Support. Either use the form or scroll to the bottom of the page in the 2nd link. Follow through the screens to get a chat or telephone option.

iTunes Support

iTunes Support – world wide

When you have a card on an account it has to be a valid card and pass the iTunes validation, that applies to downloading app updates as well as buying paid-for items.

If the card is a debit card that might be why it's being declined, not all seem to still be accepted by iTunes. Try a different card, or see if you get the 'none' option so that you can remove it : 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).