I set up family sharing. Why can't I share my iTunes library?

Why can't you exactly? What happens when you try to share?

Have you invited your family members to be part of the family sharing group?

Have they accepted?

Have you turned on Purchase sharing? link-> How to share purchases with your family - Apple Support

Are they and you following the steps here to find your purchased content? link-> Download family members’ purchases on iPhone - Apple Support

Do note Family Sharing only shares purchases content from the iTunes and App Stores. Any content obtained from other sources will not be shared.

The person needs to follow the steps in the last link to find your purchases. Here it is again: link->Download family members’ purchases on iPhone - Apple Support

If he's not using an iPhone, iPad or iPod, then tell us what device he is using and we can offer specific instructions for it.