The element of the Web.config file does not have a 'targetFramework' attribute. Therefore ASP.NET assumes that the Web application ...

The  element of the Web.config file does not have a 'targetFramework' attribute. Therefore ASP.NET assumes that the Web application targets the .NET Framework version 3.5 or earlier, but ASP.NET could not find files that are installed with the .NET Framework version 3.5 that it needs to compile. If you are compiling this Web application for version 3.5 or earlier of the .NET Framework, make sure that version 3.5 of the .NET Framework is installed. If you are compiling this Web application for version 4 or later of the .NET Framework, add the 'targetFramework' attribute to the  element of the Web.config file (for example, '').