Hi genesisgenesis,
Welcome to Apple Support Communities. We understand that you're having an issue with sharing your Apple Music Family Subscription. We'd like to help.
Does your family member who can't access Apple Music, see that they're part of Family Sharing when they go to Settings > Tap you name > Family Sharing? We'd like to confirm that they see this on their device so we know that it's set up correctly on their end.
If Family Sharing is showing on their device, have them follow these steps:
1. Open the Apple Music app or iTunes.
2. Go to Listen Now or For You.
3. Tap or click the trial offer, (one trial per person or family).
4. Choose Family, then tap or click Start Trial.
5. Sign in with the Apple ID and password that you use to make purchases. If you don’t have an Apple ID, choose Create New Apple ID, then follow the steps. If you're not sure if you have an Apple ID, we can help you find out.
6. Confirm your billing information and add a valid payment method.
7. Tap or click Join.
Let us know if that helps.
Cheers!