Hi printspat,
From what I gather, you have a payment method selected for your iTunes Store account which isn’t being accepted when setting up Family Sharing.
Take a look at the he article linked below provides, which provides a lot of great information about Family Sharing, and make sure you're trying to use a valid payment method for setting up Family Sharing.
Set up Family Sharing - Apple Support
One adult in the family—the family organizer—can set up Family Sharing for the group from their iOS device or Mac. When you set up Family Sharing, you'll be asked to confirm that you agree to pay for purchases initiated by the family members you invite and that you have a valid payment method on file. Valid payment methods for setting up Family Sharing include credit cards and debit cards.*
Cheers,
-Jason