Windows Universal Apps: “The page name does not have an associated type in namespace, Parameter name: pageToken”

When building Universal Windows Apps, and using Prism to compose the application, you would have changed your App.xaml.cs class to something like this:

public sealed partial class App : MvvmAppBase
    {
        #region Private fields

        private readonly IUnityContainer container;

        #endregion

        #region Constructors

        public App()
        {
            this.InitializeComponent();
            container = new UnityContainer();
        }

        #endregion

        #region Overrides of MvvmAppBase class

        protected override Task OnLaunchApplicationAsync(LaunchActivatedEventArgs args)
        {
            NavigationService.Navigate("Main", null);
            return Task.FromResult<object>(null);
        }

        #endregion

        #region Unity Container methods

        protected override Task OnInitializeAsync(IActivatedEventArgs args)
        {
            container.RegisterInstance<INavigationService>(NavigationService);
            ViewModelLocationProvider.SetDefaultViewModelFactory((viewModelType) => container.Resolve(viewModelType));
            return base.OnInitializeAsync(args);
        }

        #endregion
    }

But when you try to run your application you would get the error The page name does not have an associated type in namespace, Parameter name: pageToken

The reason is simple :). Your MainPage (and all views for that matter) needs to be inside the Views folder inside your project and not in the root. This applies to both Windows and Windows Phone app projects.