Unable to use apple pay

Apple Pay and iTunes vouchers are 2 different things. If there were something wrong with Apple Pay, it would have no effect on iTunes vouchers.

What exactly are you trying to do?

You cannot redeem an iTunes Voucher or gift card into the Wallet app to be used through Apple Pay, and you cannot use Apple Pay through the iTunes or App Stores which would use an iTunes Voucher.

What exactly are you trying to do?

click here ➜ How to redeem your Apple Gift Card or App Store & iTunes gift card – Apple Support (UK)

What happens when you try to redeem the voucher by following the steps in the link above? How to redeem your Apple Gift Card or App Store & iTunes gift card – Apple Support (UK)

What happens when you try to register your card then? Add a payment method to your Apple ID - Apple Support

Do you get an error? If so, what does it say exactly, in either case.