The configuration section 'system.web.extensions' cannot be read because it is missing a section declaration

This error is due to version of application pool. You need to update version of app pool from v2 to v4. Follow the below steps:

1. Open IIS.
2. Right click website and click settings to check the application pool name of website. OR
3. Click website, click advance settings in right side panel.
4. Now click application pool.
5. Double click website's application pool(or right click and properties of app pool).
6. Change version in dropdown from v2 to v4.