Apple doesn't make it clear you need to convert your Google YouTube account to a legacy account to log in on the Apple TV.
If you have an Apple TV and have ever tried to log into your YouTube account, you have probably pulled your hair out in frustration because Apple does not support Google account logins. If you have created a YouTube account in the last few years, your account is most likely a Google account and takes some effort to login to Apple TV.
When you get the message "this account is not supported" you need to persuade YouTube to create a legacy login for you.
The way you do this is to create a channel and go to your account overview to create a username.
This view should show your name above your email and if you click you name it will ask you to convert your channel to be a username.
Creating a username will allow you to login to Apple TV with this username and your Google account password.
Hopefully this will help somebody out since Apple does not make it clear you have to jump through so many hoops.