Can't add Google account to secure folder

[gsxdsm]

Can't add a Google account that is already on the device to a secure folder. Can't access Play or installed apps licenses. 

[carlosbxx]

I use my own domain so I couldn't use just the username without using the @Gmail.com. IInstead I chose the option saying I didn't know my account and used phone number and name/surnames to log in which added the account and it's all good now.

None of the other options worked.