How do I join Apple Music after joining a family subscription?

More than likely the Apple Music is an Individual plan. When joining Family Sharing, the organizer of the plan can share their subscriptions with others, but Apple Music requires the Family plan to share, which does cost more than the Individual Apple Music plan.

For more information and steps to upgrade to the Family Apple Music plan, follow this Support Article.

Get an Apple Music family subscription - Apple Support

You need to type in the apple id and pay for Apple music after you have joined the family subscription. It must have been paid for on the Family subscription.