Make sure eveything is being done as instructed in Set up Family Sharing - Apple Support
Read the section: "Invite people to join your family"
Also make sure you are not violating some basic requirement for Family Sharing such as inviting a person in a different country or somebody who has joined too many families in one year. Although the message is vague, if you persistently see that kind of message it is unlikely to be a server error on Apple's part but some basic issue blocking the person from joining.
Sometimes it helps to have the devices involved take a fresh look at settings:
- Try signing out of accounts and back in again.
- Restarting is a simple step that can resolve many unexpected behaviors.
If a problem persists, particularly over several days, try contacting Apple for assistance:
Click here --> Choose your country or region - Official Apple Support Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
"See a list of Apple phone numbers around the world."
Click here --> Contact Apple for support and service - Apple Support