Universal Family Member Can't Make In-App Purchase

[NightFox]

I have a family Apple account set up, and I'm the 'key' account holder. My (adult) daughter wants to subscribe to an app via in-app purchase, but when she tries to do that it says she needs to verify payment information. The only card details shown are the last 4 digits of my registered cards (as expected), but it doesn't show any option how she's meant to 'verify' these cards, even the 'add another payment method' is greyed out.

I know both these cards already work fine on my Apple account, so can anyone tell me what she needs to do to initiate an in-app purchase? I can't do it under my account because the IAP doesn't support family sharing.

Thanks!