My family share plan is not allowing my wife to use her Apple Music

Hi loco4,

We'd love to see if we can help resolve the issue for you. Is she able to use other shared apps without issue? If not, you family sharing may not be set up may not be complete. Check out the information below for additional information.

"If you already set up Family Sharing

After you set up Family Sharing, eligible subscriptions are automatically shared with your family members. If your family members can't access a subscription, here's what to do:

1. On your iPhone or iPad, go to Settings, then tap your name.
2. Tap Subscriptions.
3. Scroll down and make sure that Share with Family is turned on. 
4. Tap the subscription that you want to share.

• If the subscription is individual or student, upgrade your subscription to a family plan. 
• If the subscription isn't made by Apple, it might not be eligible to share. To share a subscription, turn on Share with Family under Family Access. 

If you haven't set up Family Sharing

1. Open the Settings app.
2. Tap your name, then tap Family Sharing.
3. Tap Set Up Your Family, then follow the onscreen instructions and invite your family members. Already set up Family Sharing?
4. Eligible subscriptions are automatically shared with your family members.

Learn how to use a Mac to set up Family Sharing"

Reference: Add a family member to your shared subscriptions - Apple Support

Cheers!