Hello Gary Pipe, Thanks for posting this in our community.
Please make sure the card you are trying to use is active, not expired, and supports international or online transactions. Also confirm that the billing address matches the one on your Microsoft account.
Sign in to Microsoft Account Payment Options and remove any outdated payment methods before trying to add the new card again.
If you are still having trouble, try using a different browser or clear your cache and cookies. You can also attempt to add the card through the Microsoft Store app or on a mobile device.
If cards continue to fail, consider adding PayPal or using a Microsoft gift card as a temporary solution.
If none of these steps work, I recommend contacting our Microsoft Billing Support team for direct assistance. You can reach them here: Microsoft Support Contact, then select Billing & Subscriptions and choose Chat.
Kind regards,
John Oli