I presume this is a minor age child? The document Move a child to another group using Family Sharing - Apple Support says: "You can't remove a child under 13 years old [see the document for regional age variations] from your family group, but you can move them to another group or delete their account." If this is a non-minor child you can remove them from your group as outlined in this document Leave Family Sharing - Apple Support
I am equally confused by that message so I'll direct you to Apple who can hopefully sort it out. ;-)
Contact Apple, click here --> 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