We once had to create a database on our MS SQL Server to sync with a third party site to share data between companies. In the process, I created them some SQL users in the database, but they were having issues logging in. They would get an “Error: 18456” when attempting.
Although I could’ve sworn I put it in mixed Authentication mode, the server was in “Windows Authentication Mode” which doesn’t allow SQL user logins. To fix it you just login to SSMS, right click on the server => Properties => Security => and select the “SQL Server and Windows Authentication mode” radio option.
Run => services.msc => SQL Server Service => Restart. NOTE: The service name may be the name of your instance.