While it is possible to create and use an AppleID account without a registered payment method, you’ll not be able to download or update any Apps - paid, free or otherwise - without some form a registered payment method.
Many so-called “free” Apps either have in-App purchases, or require activation of a service subscription - both of which require a payment method to be associated with your AppleID account.
See >>> Change, add, or remove Apple ID payment methods - Apple Support
If you only have a single Credit Card set-up for your AppleID account, you will need to add a new Credit Card to your AppleID account - after which you will be able to delete the old card details.
It is often a good idea to have two (or more) payment cards associated with your 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
In summary - consider this…
In context of App licensing, even “free” third-party Apps are “purchased” through the App Store - the “purchase” is both assigned and permanently recorded against the AppleID from which the purchase was made. Apple’s e-commerce system does not allow “purchases” without a valid payment method, even for transactions of zero value.