Xamarin Android Build Failed "Invalid value for outputAssembly"
If you are using the portable class library (PCL) approach to organize your code between Android and iOS, you can unload the Android project by right-clicking it and selecting menu option "Unload Project" in Visual Studio. Then recompile the PCL project, reload the Android project again (right click -> "Reload Project") and rebuild the solution. This time it should give no error. Now try to run the app again on Android emulator.
You may have to also restart the emulator and uninstall the app from the emulator/device before being able to run it succesfully again.
You have to set your Project Linking to Sdk Assemblies Only and set support architecture to armeabi - v7 and x86.
after try to clean and rebuild the project
It work like charm.
Open project in File Explorer, delete bin and obj file from the solution, go to the solution.
Right click in droid project go to option
Select Android option set your Project Linking to SDK Assemblies Only and click advanced set support architecture to armeabi-v7 and x86.
Finally to clean and rebuild the project.