I can’t access Apple TV on family sharing even tough it is shared with me
Can’t access family sharing of Apple TV+ subscription
If you are part of a Family Sharing group, and one of the group members has an Apple TV+ subscription, then all group members have access to that subscription content.
Open the Apple TV app, sign in with the Apple ID account as used for Family Sharing, go to tv+, and start watching.
On what device are you trying to watch this?