While Install SharePoint :
IIS7 deployment - duplicate 'system.web.extensions/scripting/scriptResourceHandler' section
Your Application Pool running in .net 4.0
These section definitions are already included in the root web.config in .NET 4.0
(%windir%\microsoft.net\framework\v4.0.30319\config\machine.config) that include all the system.web.extensions declared already.
- Cleanup your web.config that includes all the section Definitions that point to .net 3.5.
- Change your application pool set to 2.0