PushAsync is not supported globally on Android, please use a NavigationPage – Xamarin.Forms

You are calling “PushAsync”:

public partial class MainPage : ContentPage
{
    public MainPage()
    {
        InitializeComponent();
    }

    private void btnCourseList_Clicked(object sender, EventArgs e)
    {
        Navigation.PushAsync(new PageB());
    }
}

but you did not start the NavigationPage, which normally is done in the App.cs class, or at least it should be started before any call to “PushAsync”:

MainPage = new NavigationPage(new PageA());