If you registered Payment Card is being declined, you will need to contact the Card Issuer.
It is often a good idea to have two (or more) payment cards associated with your AppleID account - as, in case there is a problem with your primary card, a charge can be made to the second card without risk of impacting ongoing access to your AppleID account or associated services and subscriptions. With multiple Credit Cards, you can choose the sequence in which card payments are made.
You may find these support pages to be helpful:
If you need to change or update your Apple ID payment method - Apple Support
Add a payment method to your Apple ID - Apple Support
Remove a payment method from your Apple ID - Apple Support
Payment methods that you can use with your Apple ID - Apple Support
Assuming that your payment card is supported, if you cannot resolve the problem yourself, you would be best advised to seek assistance directly from Apple Support:
Official Apple Support
You can also initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:
https://apps.apple.com/app/apple-support/id1130498044