ERROR: Cannot find all types required by the 'async' modifier. Are you targeting the wrong framework version, or missing a reference to an assembly?

Question

I have following configuration of my PC:

• Windows 8
• Visual Studio 2012
• .NET Framework 4.5

My Project Configuration is:

Answer 1

From the page you downloaded from:

This targeting pack is not required for projects targeting .NET Framework 4.5 or .NET for Windows Store apps. It is only required for projects targeting Silverlight 5 and .NET Framework 4.0. Earlier platform releases are not supported.

Upgarde to Silverlight 5?

Answer 2

There is a newer version of the Async targeting package which supports Silverlight 4. Search for Microsoft.BCL.Async in NuGet, or run install-package Microsoft.Bcl.Async –prein the Package Manager Console. The package supports SL4, WP 7.1+ and Portable Libraries (yes!)

You will find a detailed description in the BCL team's blog Using async/await without .NET Framework 4.5