Email is not an Apple ID or email is being used as an Apple ID? I am getting these errors when trying to login with my mother's email.

Can it be a bug in Apple?
Sounds like you are trying to create an apple id. The email you are trying to use already exist as an apple id so you need to create a new apple id (using a different email address) or try to login with that one. If you can't recall the password click on "forgot password?"