Declined payment method?

- 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

I know I do not need any kind of payment to download or update my apps (which are only free). That said, I have seen many posts where people are required to enter a card if only temporarily for "verification" purposes (it's hard to find out what exactly verification means). Have you tried setting your payment method to "none"?

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 or are set as the Family Organizer for a Family Sharing plan.

You can only choose None when you set your payment method up from the iTunes Store, App Store, iBooks Store, or Mac App Store. None is not an option when you use the Apple ID account page to set your payment method.

Sign in with your Apple ID - https://support.apple.com/HT204053 - "When you sign in to the iTunes & App Store for the first time, you might need to add a credit card or other payment information to help secure your account." 20170919

iTunes [Store] Support - Use this form for assistance with account and billing, code redemption, or purchase questions - https://ssl.apple.com/emea/support/itunes/contact.html