I ran into this problem the other day where I updated Xamarin on my Mac and I was playing with various solutions that involved Android projects. I kept receiving this build error:
Error: /<path>/XamarinForms1.Droid.csproj: /<path>/XamarinForms1.Droid.csproj could not import "$(MSBuildExtensionsPath)\\Xamarin\\Android\\Xamarin.Android.CSharp.targets" (XamarinForms1.Droid)
I played around a bit with different project types, thinking the problem was specific to one or several of them …