Change database login default schema
It looks like based on your error that the user you are trying to create a login for already has a login in that database with another name, so you are effectively "double mapping" them.

If you have access to SSMS you can see what their mapping is by using the Object Explorer and navigating the following path:
Security -> Logins -> Double click on the username

This should pop up a GUI, you can see the "User Mapping" in the top left hand corner, this should show you to whom the user is mapped.