NuGet will not let you install a NuGet package into a project that is targeting a .NET framework which is not included in the NuGet package.
Your project is targeting Windows 8/Windows Store (.NETCore). The Prism.Composition NuGet package has an assembly for the full .NET Framework. NuGet considers these to not be compatible which is why you are getting the error. The Prism NuGet package may be using parts of the full .NET Framework which are not available to a Windows Store application which is why NuGet is failing.
The version number is not the problem. If your project was targeting the full .NET Framework 4.5.1 then you could install Prism.Composition which has an assembly for the full .NET Framework 4.5.
In order to install a NuGet package into a Windows Store project it would need to explicitly target Windows 8 or include a Portable Class Library (PCL) which is compatible.
There are a couple of Prism NuGet packages that contain PCLs that you might be able use instead.