Add someone to family sharing

Ugh, one of those numeric errors. After trying to see if simply restarting your device clears up the issue, it's just the sort of thing to ask Apple about. 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