Had this build error – preventing all IOS development. Searched around for fixes all over the place but of course the solution was simple as usual 🙂 I had accidentally added a direct Mono.dll dependency in my base project and was referencing the Android namespace (using Android.***) in a couple of (XML and c#) files. This caused the MT2001 horror 🙂 Again, learned a lot about the build procedure during the debugging process but the solution was just toooo simple as usual. Hope this helps someone.