Transfer my minor child to another family sharing

Ensure you are doing as guided here --> Move a child to another group using Family Sharing - Apple Support

Try signing out of your account and back in again.

Try restarting your device.

Contact Apple and ask if they can tell you what this means, particularly if you see it repeatedly over a day or so:

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