Hello,
because I have a lot of trouble with keychain (ios), android project (see another post) etc...
I decided to restart from scratch, create a new solution, and add step by step the different class and object and package, in each project.
I am using the latest version of Xamarin visual studio for MACOS : 7.4
I created a new solution, Xamarin forms with ios and android project, the solution is almost blank : there is one xaml page, one code behind code.
the ios project compile, and run fine on device.
tha android version (again, I just create the solution, all is created by xamarin) give me an rror (sic !) 
/Users/ormaa/XAMARIN/Projects/cupidon/Droid/Resources/values/styles.xml(0,0): Error APT0000: Error retrieving parent for item: No resource found that matches the given name 'Theme.AppCompat.Light.DarkActionBar'. (APT0000) (cupidon.Droid)
I did modify anything the the solution or project, and create all of this in a new empty folder.
I discover the some package installed by xamarin were not up to date.
I tried to update the package, using the options on android project, it failed with that error :
Could not install package 'Xamarin.Android.Support.Animated.Vector.Drawable 27.0.2'. You are trying to install this package into a project that targets 'MonoAndroid,Version=v7.1', but the package does not contain any assembly references or content files that are compatible with that framework. For more information, contact the package author.
what can I do ?
I woud not delete xamarin, and reinstall it, my internet connection is so slow that I will lose 2 Days of work 
(
thanks
Olivier