The sp_change_users_login procedure has a specific purpose. It’s used to identify and correct users within a database which do not have a corresponding logins.
Execute the following query to see any users which do not have a corresponding login.
exec sp_change_users_login 'report'
Execute the following query where username is the username that you want to fix.
sp_change_users_login 'auto_fix', 'username'
This only needs to be done when you are restoring a database from one server to another, and the logins on each server were created with seperated SIDs. An example of when this would happen is when you are restoring database from Production to QA.
Map an existing database user to a SQL Server login. If the login for a user has changed, use sp_change_users_login to link the user to the new login without losing user permission. The new login cannot be sa, and the user cannot be dbo, guest, or an INFORMATION_SCHEMA user.
This feature will be removed in a future version of Microsoft SQL Server. Avoid using this feature in new development work, and plan to modify application that currently uses this feature. Use ALTER USER instead.