Hey gernasia,
We'll be happy to help out with setting up Apple Pay for family members under 18. Here's the steps you'll want to follow to set that up: Set up and use Apple Cash Family - Apple Support. If you are still seeing that error after following those steps, follow this guidance to manage the Apple Cash account, as Apple Cash may already be set up for your child: View and limit your child or teen's Apple Cash activity - Apple Support
Cheers!