Apple may often require the presence of a valid payment method (see the link at the bottom) even if something is free.
This document explains how to 'Change, add, or remove Apple ID payment methods' on an Apple mobile device, a Mac, or a PC; click here --> https://support.apple.com/HT201266
- If you are updating an old card and have ongoing subscriptions you may need to first add the new card and make it the primary payment method before deleting the old card.
Review this document about what to do "If your payment method is declined in the App Store or iTunes Store" Click here --> https://support.apple.com/HT203005 It mostly just suggests you make sure your payment method is up to date or to try a different one. Apple is probably just checking with your financial institution if the payment method is valid and getting a negative response of some kind. Make sure the name and address are entered exactly as it appears on your bills and that the 3 digit CCV is correct. Check with your financial institution about the status of your card.
Make sure this is a payment method that you can use with your Apple ID; click here --> https://support.apple.com/HT202631
If you still have problems Click here --> https://support.apple.com/choose-country-region/media-services Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
Contact Apple for support and service --> "See a list of Apple phone numbers around the world." - https://support.apple.com/HT201232