Could not resolve this reference. Could not locate the assembly "mscorlib". Check to make sure the assembly exists on disk. If this reference is required by your code, you may get compilation errors.
I tried cleaning build, restoring project , updating NuGet packages but nothing is working
This things worked for me:
Android : Set AndroidHttpClientHandlerType to Xamarin.Android.Net.AndroidClientHandler
iOS : In iOS i have deselected "Do not reference mscorlib.dll"
In Project properties settings.