When trying to use the ConfigurationManager in C#, Visual Studio doesn’t recognise that ConfigurationManager exists, even though you’ve added “using System.Configuration”.
If ConfigurationManager does not exist, it’s because you haven’t added a reference to the System.Configuration.dll assembly. To fix this, right click the project -> Add -> Reference, then tick System.Configuration. Visual Studio should not report that ConfigurationManager does Not exist anymore (assuming you’ve added “using System.Configuration”.