If you are using SQL Server authentication to connect to a destination database, you need to ensure that the user has been granted "dbo" access to the database.
If that is not done, you will get the error: Login failed for user '<username>'.
To do that, follow these steps:
1) In SQL Server Management Studio (SSMS), expand the nodes Security -> Logins, then right-click the username and select Properties;
2) In the property window, select User Mappings, then select the destination database and ensure it is checked, ensure the "db_owner" role is selected, and finally press OK to save the changes.
That's all, now the user has full access to the database and can be used in SQList for the authentication.