latest.log

I'm not sure if Microsoft changed the api or what but I'll look into it. Thanks for the report

I believe this was an issue on Minecraft's side after doing some digging. Can you try again and see if the issue still happens? Thanks.

I tried again and unfortunately, it still didn't work, if this helps the account I was adding was a different account than I was signed into on my client.
latest.log

Is the accounts date of birth under 18 years old?

Sadly their api doesn't allow accounts under 18 to use 3rd party logins for some reason. So I believe you either need to change the age if you can or setup a family group on an adult account. In the next update I'll add a warning to let people know

Yes.