System.ObjectModel Warning Xamarin does not run on Android
Right click on your Android project's references and manually add the reference: C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\MonoAndroid\v1.0\Facades\System.ObjectModel.dll. And technically that's a warning, not an error.
I simply removed the reference entirely since none of my code actually used it.
In the Package manager console type: (Select the Android project)
update-package -reinstallIs slower but is the correct method and always works