Family Subscriptions

Re: "My family subscription will not apply to my Apple Music app"

You may have an individual Music subscription.

It seems that only a family Music subscription can be shared using Family Sharing.

What types of content can I share with my family? - Apple Support

If you have a Family music subscription (not just an individual subscription) everyone in Family Sharing automatically has their own Library to which they can add whatever music they wish to listen to.

Apple Support's article: Get an Apple Music family subscription - Apple Support has details,

& says:

"With an Apple Music family subscription, up to six people can enjoy unlimited access to Apple Music on their devices".

If needed: How to switch to a different subscription plan - Apple Support

Family Sharing website:

Family Sharing - Apple