Is that device very old? Apple used to say this in one of its documents: "You can use Family Sharing on your iPhone, iPad, or iPod touch with iOS 8 or later, your Mac with OS X Yosemite or later and iTunes 12, or your PC with iCloud for Windows."
That's all I can think of and you many need to ask Apple what that message means:
Refer to this document for ways to contact Apple --> Choose your country or region - Official Apple Support
Select your country (also look for "other" regions), 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:
Contact Apple for support and service by telephone --> Contact Apple for support and service - Apple Support