Some times we may get the below error while updating the login from dbo to some other login from user mapping window.
1. I have database which is mapped with dbo login and dbo schema.
2. I tried to update the database with app_user login, then it displays that error message.
To fix the issue we need to change the default owner to 'sa' and then tried to update the login.
It may cause because dbo is mapped with some other login, then we need to change the dbo login name to 'sa', it is because the sa login is reffered as dbo and it will exists in all the databases. So we should not modify the dbo login to any other login except 'sa'.