An ASP.NET setting has been detected that does not apply in the Integrated managed pipeline mode

Running the following command.

%SystemRoot%\system32\inetsrv\appcmd migrate config "Default Web Site/".

and then  refreshing the webpage and everything will work perfectly!

Another thing we can try is that , changing the web's application pool from "Default applicaion pool" to FrameWork 4.0 classic also do the trick some times.