It’s not letting me start family sharing?

It would help a lot if you told us why you can't. Is there an error message? What makes you think you can't? If you call a car repair service and say you cannot start your car and that's all you say, they don't know if you have lost your key or if the entire engine was stolen. ;-)

To learn how to 'Set up Family Sharing', click here --> Set up Family Sharing - Apple Support