An Apple Community Specialist in this topic about a payment method being declined in ApplePay: https://discussions.apple.com/thread/252666303 recommends reading this support article: If your payment method is declined in the App Store or iTunes Store - https://support.apple.com/HT203005 This article mostly suggests is if a payment method does not work, try a different one. Realize that Apple is likely just checking with your financial institution if the payment method is valid and getting a reply that something is inconsistent with their records. Check with your financial institution about the status of your card. Also check your payment information you are entering exactly matches what you get on bills from your card holder.
To contact Apple about this:
Click here --> https://support.apple.com/choose-country-region 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."
Click here --> https://support.apple.com/HT201232
