Connecting to Microsoft SQL Server from QGIS

I would check the name of the MSSQL server again. Normally it is something like: username/MSSQLSERVER unless you renamed it during installation. I would also either set the checkbox to trusted connection or enter the sa user if you have the rights. Before you can test the connection, you have to click on the database list. Make sure that you have checked the box at the bottom for "Also list tables with no geometry, because especially in the beginning you often don't have any geodata in the database. Select your database and you can test the connection.