Hi Terrace,
Welcome to the Apple Community.
Apple support has an article online that may help:
If you can't add a card to Apple Wallet to use with Apple Pay - Apple Support
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If you need more help:
Wondering where it is you are starting from, to enter the card details
For iTunes, Apple Music, or App Store: see below for your Apple ID account
For your Apple ID account (ie: purchases from Apple):
Try entering the details online in your account:
Here's the link to Apple ID log-in:
https://appleid.apple.com
For Apple Pay (ie: purchases from merchants)
Apple Support's article:
Set up Apple Pay
says:
In the Wallet app, tap the Add button (+).
Tap Debit or Credit Card.
Tap Continue.
Follow the steps on the screen to add a new card.
If prompted, choose your bank or card issuer from the list or find them using the search bar.
Verify your information with your bank or card issuer. They may ask you to provide additional information or to download an app before approving your card for use with Apple Pay.
If needed: There is more info. in the above linked article.
Also:
How to: Make purchases with Apple Pay
Here's a link to the Apple Pay Support Website
(There is a "Get Support" link at the bottom of their webpage.)
Apple Pay - Official Apple Support
All the best :-)