Apple store

Go to Settings > iTunes & App Store > Apple ID and check your payment information is current. Even if you only want something that is free this will be required if you already have any payment form on record.

- 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.

- 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

Worldwide iTunes support - https://support.apple.com/country-selector/itunes

Alternatively for free apps read this article about setting your payment to none if it will allow it:

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

What exact error message?