Navigation Confusion

I am obviously missing something or not understanding the Navigation correctly. I have a UserLogin page that when the user logs in they are sent to another page for two factor authentication.

await Navigation.PushModalAsync(new TwoFactorPage(UserKey, TicketValue));

this works fine and it the user get the two factor correct I send them onto the main page

await Navigation.PushModalAsync(new MainPage(UserKey));

But if the user gets the two factor incorrect I want to send them back to the userlogin page but everything I do results in a error.
I thought I was supposed to use

await Navigation.PopAsync();

but that doesn't work, give the me lovely "is not supported globally on Android, please use a NavigationPage"

So am I doing my page navigation wrong? How do I go back a page?