- Are you using a valid payment method? --> Payment methods that you can use in the iTunes Store, App Store, and iBooks Store - https://support.apple.com/HT202631 (Discussions on this forum mention some countries do not accept debit cards, and even if they do some items cannot be purchased with debit cards.)
- If your payment method is declined, or you have a billing issue with a previous purchase in the iTunes Store - https://support.apple.com/HT203005 - various reasons why payment method is declined.
- If this abruptly started happening try removing and re-entering your payment information - Change or remove your Apple ID payment information - https://support.apple.com/HT201266
- Do the name, address (all spelling, letter case and punctuation), and card numbers exactly match those you have entered? You didn't mis-type anything?
- You entered the 3 digit security code for the card? Correctly?
- The country for the bank and your address are the same as the iTunes Store country you are using?
- See the end of the document linked above for special measures that may be necessary for some cards, plus about contacting your financial institution to make sure your records match.
If you still have a problem contact Apple:
iTunes Store - Contact Us - https://www.apple.com/emea/support/itunes/contact.html