Iphone 7+ Can't add card to apple pay

You don’t say what specifically is the issue (do any error codes or warnings appear) or whether you tried looking at Apple’s help articles, so just in case.

If you can't add cards to Wallet for Apple Pay - Apple Support

Set up Apple Pay - Apple Support

Note the small print for conditions and restrictions, also sometimes answers are buried in other text so reading through a couple of time may help.

Have you tried to delete the existing card info, then add the new one?

Regards

Giulio