How to recover an Apple account when I get a "apple ID is either not valid or not supported" message?

I'm trying to recover access to an Apple account. Basicaly I know the ID, but not the password and I have access to the email address (outlook.com in this case)

On iforgot.apple.com, after entering the apple ID , i received the following message:

"Apple ID is either not valid or not supported"

Solution 1: not valid: seems strange as this apple ID is used on several devices

Solution 2: not supported; I do not understand what it means and entails.

Does anyone know how to recover an account when such messages appear?

Thx !