It's a paid service, so a payment method needs to be set up. Follow these steps to set up Family Sharing on Apple Music. As noted in the link:
One adult in the family—the family organizer—can set up Family Sharing for the group from their iPhone, iPad, iPod touch, or Mac. If you turn on purchase 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.*Â
Good luck & happy streaming!
-- JDee