C#, asp.net - The name 'ConfigurationManager' does not exist in the current context

This is due to System.Configuration.dll, Reference to this dll is missing in project and you are using this class(which is in System.Configuration.dll). To resolve this you need to add reference of System.Configuration.dll.

Right click References in Solution Explorer.

Add Reference.

In Assemblies > Framework, browse to System.Configuration.dll.

Check the checkbox.

Click OK.