Unrecognized attribute 'targetFramework' - ASP.Net 4.0

This is generally associated with the wrong .NET Framework defined in the Applicaiton Pool for which the website is running.  To resolve this, do the following:

1.  Go to the Control Panel.
2.  Double-click on Administrative tools.
3.  Double-click on Internet Information Services (IIS) Manager.
4. Expand the server and click on Application Pools.
5.  Look at the .NET framework for your website. 
6.  Right-click on the website and select Basic Settings.
7.  Change the .NET Framework version to .NET Framework v4.0.30319 or whatever version 4.0 you have.
8. Click OK.