The type or namespace name 'Xamarin' could not be found

i've Created a xamarin Cross-platform solution (Blank) and it runs normally on android emulator whenever i try to run it on xamarin Live Player i get the below errors

1- AppDelegate.cs(1,1): error: The type or namespace name 'Xamarin' could not be found in the global namespace (are you missing an assembly reference?)
2- AppDelegate.cs(1,1): error: 'AppDelegate.FinishedLaunching(UIApplication, NSDictionary)': no suitable method found to override
3- AppDelegate.cs(1,1): error: The name 'LoadApplication' does not exist in the current context
4- AppDelegate.cs(1,1): error: The type or namespace name 'App' could not be found (are you missing a using directive or an assembly reference?)

i'm running VS 2017 ENT 15.5.6 on Windows 10 Pro. and everything is updated