On a mobile device go to Settings > iTunes & App Store > Apple ID and check your payment information is current. Even if you only want something that is free Apple will insist your current payment method can be verified with your financial institution. You can try removing payment information (see the last two links) to avoid the problem with the existing method but it is possible if you have any payment form currently on record this must be resolved first before you can do anything.
- Are you (still) 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 including pending purchases.
- 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
To remove payment:
Removing a credit card from an account - Change or remove your Apple ID payment information - https://support.apple.com/HT201266 - More information at: https://discussions.apple.com/thread/3251676?answerId=15891166022#15891166022
If you can‘t choose None when you edit your Apple ID payment information - https://support.apple.com/HT203905 - also make sure you don't have any active subscriptions, are set as the Family Organizer for a Family Sharing plan, or owe any money for past purchases.
