- Are you [still] using a valid payment method? --> Payment methods that you can use with your Apple ID - (Prepaid cards may not work or may only work erratically. Cards that previously worked may not always work.) - https://support.apple.com/HT202631
- Read this link about what to do "If your payment method is declined in the App Store or iTunes Store" - Click here --> https://support.apple.com/HT203005
- If this abruptly started happening try removing and re-entering your payment information - Change, add, or remove Apple ID payment methods - 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 expiration date and 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 declined payment 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. I suspect Apple declining a payment method is just passing on a message that there is a problem with the card at the financial institution.
If you still have a problem contact Apple:
Click here --> https://support.apple.com/choose-country-region/media-services Select an option and keep on working your way down the menu until you get to one saying your "topic isn't listed". This will present you with support by chat or telephone call.
or
Worldwide iTunes support - https://support.apple.com/country-selector/itunes