Why can I not log into my Apple Music when we have family sharing

Seems Apple Music subscriptions are often either individual or family subscriptions.

Only the family subscriptions are accessible with Family Sharing,

although some iTunes Music purchases may be family-shared

If needed: Learn how to change to a family subscription.

or

Get an Apple Music family subscription - Apple Support

Eg: says:

"If you already set up Family Sharing, everyone in your family group will automatically have access to Apple Music after you subscribe"