Sorry excuse me, I don’t know why I can’t join the Apple Music subscription through the family member and it really confused me.

Hello Icreamlover123,

1. Open Settings
2. Tap Family
3. Tap Subscriptions

Is Apple Music being shared? This section shows all subscriptions being shared with the entire Family Share group.

I don't know if you need to be an organizer to do this (I've never been on the other end).. Try this below and if it doesn't work have the organizer check!

1. Open Settings
2. Tap Family
3. Tap Your Name
4. Tap Subscriptions

This will show you what subscriptions you have access to.

If it's not being shared, whoever bought the subscription (needs to be an Apple Music plan that supports Family Share), needs to do the following - Share Apple and App Store subscriptions with family members on iPhone - Apple Support

Share Apple subscriptions

1. Go to Settings  > [your name] > Family Sharing.
2. Tap Subscriptions, then do one of the following:

• • Tap a subscription you want to share, then follow the onscreen instructions.
  • Tap Manage Subscriptions, then tap a subscription.

You can also do this another way:

1. Open Settings
2. Tap Your Name (Apple ID) - Individual who bought the Apple Music plan
3. Tap Subscriptions
4. Toggle on Share with Family