Setting up apple pay

The message is indicating an error in matching a credit/debit card to her account. Is she under 13 and does she have a child's Apple ID? If this is the case she will be unable to use Apple Pay with her Apple ID.

See footnote 2 in Set up Apple Pay

If you're under the age of 13, Apple Pay isn't available and you can't add a card in the Wallet app. Age varies by country or region.

If she is 13 or over you could contact Apple Support - choose Show All > Apple Pay > Bank Cards and Wallet.

You're very welcome.

If you haven't already set one up, a Family Sharing group with purchase sharing will enable her to buy content on her phone with your card and, using Approve what kids buy with Ask to Buy, with your permission. See What is Family Sharing? and a little way down that page the link How to share purchases with your family