Encounter this error today while using Xamarin.
The application could not be started. Ensure that the application has been installed to the target device and has a launchable activity (MainLauncher = true). Additionally, check Build -> Configuration Manager to ensure this project is set to Deploy for this configuration.
The setup was as follow: Using the Xamarin plugin for Microsoft Visual Studio 2012 in Windows 8, debugging on a Galaxy Nexus. The problem started when switching to a Nexus 7. Problem stayed when going back to the mobile, which doesn’t make much sense as nothing else has changed manually.
However, to solve it, open Build -> Configuration Manager
Check Deploy on your project. Now Run your application again and the issue should have been resolved.