Project not Built in Active Configuration - Xamarin/Xamarin.Form
In Xamarin Studio (I'm using 5.10.3), click on Project -> Active Configuration -> Debug | iPhoneSimulator
SOLUTION
Options --> Configuration-->Configuration Mappings-->ProjectName.iOS-->Debug | iPhoneSimulator
Make sure to set it to Debug | iPhoneSimulator and NOT to Debug | iPhone
Now you could able to see Set As Startup Project option.