User283571144 posted
Hi JagjitSingh,
According to your description, I suggest you could firstly open the references tag in your project and make sure you have already added the System.Configuration
reference.
Like below:

If you don't find this reference, I suggest you couold follow below steps to add the reference.
Notice: Make sure your .net farmework version larger than 2.0
- Right-click on references tab to add reference.
- Click on Assemblies tab
- Search for 'System.Configuration'
- Click OK.
Best Regards,
Brando