I think your connection string is not written properly.
It should be :
<br />
connetionString="Data Source=YourServerName;<br />
Initial Catalog=YourDatabaseName; User ID=UserName;Password=Password"
Incase you are using Windows Authentication you just have to remove User Id and Password and add
Integrated Security=True
You can go through below link for more detail :
C# SQL Server Connection[
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