My application does not give an error, but nothing appears on the screen (xamarin)

Developing and learning xamarin. I took an example of Macoratti and decided to implement, because it is consumption of REST with xamarin. I did everything as it is on the author's website and when I squeeze, nothing appears on my smartphone (samsung J5). When I leave the first StackLayout which is a label, it appears. But the other StackLayout shows nothing. Inside the first stack, I put a Entry component together with the label, it appeared, but the placeholder was cut in half horizontally, ie only the bottom half appeared. I put below my code: MainPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:Teste2"
             x:Class="Teste2.MainPage">

    <StackLayout Orientation="Vertical">
        <StackLayout Padding="5,5,0,0">
            <Label Text="Adicionar um Produto" TextColor="Green" />
        </StackLayout>
        <StackLayout Padding="10,0,10,0">
            <Entry x:Name="txtNome" Placeholder="Nome do produto" HorizontalOptions="Start" 
                    VerticalOptions="StartAndExpand" HeightRequest="30" WidthRequest="300" FontSize="Small"/>
            <Entry x:Name="txtCategoria" Placeholder="Categoria do produto" HorizontalOptions="Start" 
                    VerticalOptions="StartAndExpand" HeightRequest="30" WidthRequest="300" FontSize="Small" />
            <Entry x:Name="txtPreco" Placeholder="Preço do produto" HorizontalOptions="Start" VerticalOptions="StartAndExpand" 
                    HeightRequest="30" WidthRequest="300" FontSize="Small" />
            <Button HorizontalOptions="FillAndExpand" VerticalOptions="StartAndExpand" HeightRequest="40" Text="Adicionar/Atualizar Produto" 
                    Clicked="btnAdicionar_Clicked" FontSize="Small"/>
        </StackLayout>

        <!--<StackLayout Orientation="Vertical" Padding="10,5,10,0">
            <ListView x:Name="listaProdutos" ItemSelected="listaProdutos_ItemSelected" BackgroundColor="Aqua" SeparatorColor="Blue">
                <ListView.ItemTemplate>
                    <DataTemplate>
                        <ViewCell>
                            <ViewCell.ContextActions>
                                <MenuItem Clicked="OnAtualizar" CommandParameter="{Binding .}" Text="Atualizar" />
                                <MenuItem Clicked="OnDeletar" CommandParameter="{Binding .}" Text="Deletar" IsDestructive="True" />
                            </ViewCell.ContextActions>
                            <StackLayout Padding="10,10" Orientation="Horizontal">
                                <Label Text="{Binding Nome}" HorizontalOptions="StartAndExpand"/>
                                <Label Text="{Binding Categoria}" TextColor="Blue" HorizontalOptions="Center"/>
                                <Label Text="{Binding Preco}" HorizontalOptions="End"/>
                            </StackLayout>
                        </ViewCell>
                    </DataTemplate>
                </ListView.ItemTemplate>
            </ListView>
        </StackLayout>-->
    </StackLayout>

</ContentPage>

MainPage.xaml.cs

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

App.cs

public partial class App : Application
    {
        public App()
        {
            InitializeComponent();

            //MainPage = new Teste2.MainPage();
            MainPage = new NavigationPage(new Teste2.MainPage());
        }

        protected override void OnStart()
        {
            // Handle when your app starts
        }

        protected override void OnSleep()
        {
            // Handle when your app sleeps
        }

        protected override void OnResume()
        {
            // Handle when your app resumes
        }
    }

EDIT1 I did this and I can see my Entry, but what I write or the placeholder is cut in half, that is, I only read the bottom. I can not read the whole text. What can this be? However, if you create more than StackLayout , there does not appear anything in the App. It seems that you are not accepting% nested%.

StackLayout Padding="10,0,10,0">
            <!--<Label Text="Adicionar um Produto" TextColor="Green" />-->
                <Entry x:Name="txtNome" Placeholder="Nome do produto" HorizontalOptions="Start" 
                    VerticalOptions="StartAndExpand" HeightRequest="30" WidthRequest="300" FontSize="Small"/>
            <Entry x:Name="txtCategoria" Placeholder="Categoria do produto" HorizontalOptions="Start" 
                    VerticalOptions="StartAndExpand" HeightRequest="30" WidthRequest="300" FontSize="Small" />
        </StackLayout>