Error System.Reactive.Unit

I get this message when I try to Debug the iOS version of a Xamarin Forms project on VS on a Mac.

I have not been able to find a solution in any forum.

/../iOS/MTOUCH: Error MT2002: Failed to resolve "System.Reactive.Unit" reference from "System.Reactive.Core, Version=3.0.0.0, Culture=neutral, PublicKeyToken=94bc3704cddfc263" (MT2002)

Thanks for any help.