Problem with family sharing Apple Music

As stated there is no support for iOS beta. You are the bug tester and need to report any bugs using the Feedback method agreed to when choosing to become a tester. Review the information here about all beta software:

Apple Beta — FAQ

• Don’t blog, post screenshots, tweet, or publicly post information about the beta software, and don’t discuss the beta software with or demonstrate it to others who are not in the Apple Beta Software Program.
• Please note that since the beta software has not yet been commercially released by Apple, it may contain errors or inaccuracies and may not function as well as commercially released software. 
• We encourage you to submit feedback when you encounter these issues. If you need further assistance, please note that you must restore to previously-released software prior to contacting AppleCare.

The subscription to Apple Music must be the Family subscription that costs more than the Individual plan. With the Individual plan, the other family members will not be able to access Apple Music in Family Sharing.

If using the iOS 18 beta, you can expect some features to not work and support is not available. For problems using the Beta, you are directed to use the provided Feedback when you chose to be a bug tester. For support, it is recommended to restore to the shipping version and verify if the issue persists.

Apple Beta — Restore Shipping Software