According to Apple Docs here, you are allowed to create sandbox users with an email sub-address of your base email address Apple ID.
For example, if my Apple ID email address is [email protected], I could create a sandbox user with [email protected].
However, when I tried to add a new sandbox user, I got an error saying This email address is not available for use as an Apple ID. You may already have an Apple ID associated with this address. Please try again or sign in using your existing Apple ID.
Does anyone know if this has changed and they just didn't update their docs yet, or perhaps I'm misunderstanding the documentation?