Can't add family member to family sharing - Cannot Complete Action error

Hi all

I am trying to add my son to our family account. I am the organizer and have successfully managed to add my other two children. The third account doesn't seem to want to add for some reason, I get to the Share Location option, select the option to "Share Your Location", click next and then an error message comes up saying Cannot Complete Action with further detail saying that the account name entered is invalid.

All three iCloud accounts were set up in the same way, they have all had purchases completed from the accounts in the past and then I have created a Family Sharing account. Success with the first two and repeated failures each time with the third. I opened a ticket with Apple Support back in December, but haven't heard back...anybody any ideas?