This error typically pops up whenever there is an attribute of targetFramework="4.0you’re your web.config but on the other hand the App Pool is pointing to run ASP.NET 2.0. In the current situation the targetFramework attribute is completely unrecognized by ASP.NET 2.0 So just changing it to 2.0 will not have the desired effect.
You need to Contact your Support or the Administrator and request them the AppPool to be switched to 4.0.
Switching your App pool to 4.0
Please check in the top level of your IIS Manager, you will see the required Application Pools tree node. Just right click on the "Application Pools" and choose the "Add Application Pool".
After that give it a desired name and choose the .NET Framework 4.0 the mode as either Integrated or Classic mode.
So now when you will add or edit your web site, the new application pools will now be showing up in the list.
If you follow above procedure your issue will be resolved..