If you still can't see the simulator devices to run on.Now close the Simulator (from the Simulator menu).Launch the Simulator manually (I have it in the Dock, so I just click it).If you can't run a Xamarin App, Close Visual Studio for Mac.either on the Mac itself or on another computer connected to the Mac (if any). Close ALL copies of Visual Studio that are connected to the Mac.So hare's something I found works that you might want to try. So, at first, I started clearing the cache with a 'xcrun simctl erase all' command from Terminal, and when I started Visual Studio for Mac I could run a Xamarin App, but launching the simulator took quite a bit of time. it ends up being simpler and quicker than anything else I have found. but you may just want to go down to the Addendum 2 (final?) and see if that works for you. You are welcome to read through all of this. 'The target name of simulated device could not be added because it's already cached'.

Looking at the Visual Studio for Mac logs, specifically the IDE log, I found this line.

Mac xamarin