Configuration Error: <compilation debug="true" targetFramework="4.0"> ASP.NET MVC3

This typically happens when you have an attribute of targetFramework="4.0" in the web.config but the App Pool is set to run ASP.NET 2.0. The targetFramework attribute is entirely unrecognized by ASP.NET 2.0 - so changing it to 2.0 won't have the desired effect.

Contact Support / Your Administrator and have the AppPool switched to 4.0.

You could also remove the attribute entirely, however if your site was coded with the 4.0 Framework, then I'm sure something else will cause an error as well.

Unrecognized attribute 'targetFramework'. Note that attribute names are case-sensitive

I was now able to find reason behind the error guys. It's on the webhosting site settings not on my app pool or what so ever. I changed the asp.net version from 2.0 to 4.0 but I forgot to click the update button. I feel so embarrased. :( I've been struggling for days to fix this error and only to found out its on the webhosting site settings. GgrrRR.. As of now I'm facing new error but has nothing to do with this question. Anyway thanks guys for your efforts to help me. :)

Try this, instead of selecting your websete's Application Pool to DefaultAppPool, select ASP.NET v4.0

Select your website in IIS => Basic Settings => Application Pool => Select ASP.NET v4.0

and you will be good to go. Hope it helps :)