Modify App.xaml and add ViewModelLocator

App.xaml

<Application
    x:Class="SimpleMVVM.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:SimpleMVVM"
    RequestedTheme="Dark"
    xmlns:vm="using:SimpleMVVM.ViewModels">

    <Application.Resources>
        <ResourceDictionary>

            <vm:ViewModelLocator x:Key="ViewModelLocator"/>
        </ResourceDictionary>
    </Application.Resources>
</Application>