You have to select your server name and the authentication mode correctly.
The default server name is your computer name, but you can also use "." (Dot) instead of the local server name.
To select your server name, follow the steps below.
Then set the correct authentication mode: Windows or SQL Authentication based on your SQL Server Installation.
If you had given SQL Authentication during the Installation of SQL Server, then select SQL Authentication, then provide the username as "sa" and give the password whatever you had given during SQL Server installation and connect.
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