Are you positive the card is valid? Use the document Add a payment method to your Apple Account - Apple Support to review your payment methods such as credit or debit cards. It it shows red, there is a problem. In that case, ask your bank or issuing agency about it.
Also review the kinds of payment methods you can use. To see the payment methods that you can use with your Apple Account [formerly Apple ID], click here --> Payment methods that you can use with your Apple Account - Apple Support
Note that these are ones that are accepted, but specific kinds (e.g., credit or debit cards) might be required in particular circumstances.
Check there are not any problems with your payment method. They will usually appear in red lettering. If there are, make sure the name and address are entered exactly as it appears on your financial institution bills and that the 3 digit CVV is correct. If everything appears correct, check with your financial institution about the status of your card.
The article: If your payment method is declined in the App Store or iTunes Store - Apple Support has some information about declined payment methods.
- Make sure your payment method is up to date or to try a different one ( If you need to change or update your Apple Account payment method - Apple Support ).
If you cannot see a reason for a problem, contact your financial institution first. Apple does not know why your card is not being verified. If your bank cannot help, contact Apple for assistance with this.
To contact Support, click on this link and choose a method —> https://getsupport.apple.com/?caller=cups&PGF=PGF63006&category_id=SC0060
Even free apps may require a working payment method be present, even if it isn't actually used.