“Declining” usually comes from your bank, so you might want to contact them.
Normally, every Account needs to have a valid payment method, even for free apps or updates. You can, however, change your payment method to None. You will not be able to do this if there’s an outstanding balance in your account.
Find out why you can't change your payment or credit card information to None in the iTunes Store, App Store, iBooks Store, or Mac App Store.