Hi MysticBlue190,
Welcome to the Apple Community.
Your Apple ID is the account you use to access Apple services,
like the App Store, Apple Music, iCloud, iMessage, FaceTime, and more.
Seems you may need to add your Mastercard to your Apple ID account.
How to: Change, add, or remove Apple ID payment methods - Apple Support
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If needed:
How to: Add card to wallet to use with Apple Pay:
How to Set up Apple Pay
says:
Add a new card on your iPhone
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.
How to Make purchases with Apple Pay
All the best :-)