This can happen when building a Xamarin project while using certain libraries. What can help is:
- Update or install the latest version of Xamarin.Build.Download NuGet.
- Close all Visual Studio windows.
- Delete all the obj, bin, .vs folders, as well as the local packages folder in your solution root.
- Delete the global NuGet cache from ~\.nuget
- Load the Solution again, Rebuild, and hopefully the problem will go away!