Q: About Subscribe and Purchasing Apps

Have you tried removing the payment card from your account and re-adding it : Change or remove your Apple ID payment information - Apple Support ? If you get a declined message when trying to add it back then what type of card is it - not all debit cards seem to still be accepted, and prepaid cards are also likely to not work. If you do get declined (and the address details on your iTunes account match, exactly, the address the card is registered to) you could try contacting the card issuer to see if they are declining it for use with iTunes, or try a different card.