What Master Card gift card?
Apple Pay holds no funds in and of itself. There's no way to add funds to it, because it is not an account, it is a payment conduit, to make payments at physical stores, restaurants and online on websites or apps that support Apple Pay.
You can add funds to your Apple ID, to pay for things from the iTunes, iBooks and App Stores, and any recurring subscriptions such as iCloud+ or Apple TV+ but that has nothing to do with Apple Pay.
click here ➜ Add money to your Apple Account balance - Apple Support
What exactly are you trying to do?
Do note, if there's an issue with your current payment method, you will need to resolve it. It will not let you use account funds until any issue with a payment method has bene resolved and it has bene successfully verified and can be charged.