UWP Simple Animation using MVVM

The problem is that UIElement.StartAnimation is available from 10.0.17763.0 and your min version is 10.0.17134.0. You can use animation through visual layer anyway.
As I do not like to have such kind of code in view model, I propose to you a different solution using Microsoft.Xaml.Interactivity.

Solution with behaviors

Extend Microsoft.Xaml.Interactivity.Behavior

public class ScaleBehavior : Behavior
{
    private readonly Compositor _compositor = Window.Current.Compositor;
    private SpringVector3NaturalMotionAnimation _springAnimation;
    private Visual _elementVisual;

    protected override void OnAttached()
    {
        if (!(AssociatedObject is UIElement uiElement)) return;

        uiElement.PointerEntered += Element_PointerEntered;
        uiElement.PointerExited += Element_PointerExited;
        _elementVisual = ElementCompositionPreview.GetElementVisual(uiElement);
    }

    protected override void OnDetaching()
    {
        if (!(AssociatedObject is UIElement uiElement)) return;

        uiElement.PointerEntered -= Element_PointerEntered;
        uiElement.PointerExited -= Element_PointerExited;
        _elementVisual = null;
    }

    private void Element_PointerEntered(object sender, PointerRoutedEventArgs e)
    {
        CreateOrUpdateSpringAnimation(1.5f);
        Vector2 size = _elementVisual.Size;
        _elementVisual.CenterPoint = new Vector3(size.X / 2, size.Y / 2, 0);
        _elementVisual.StartAnimation("Scale", _springAnimation);
    }

    private void Element_PointerExited(object sender, PointerRoutedEventArgs e)
    {
        CreateOrUpdateSpringAnimation(1.0f);
        Vector2 size = _elementVisual.Size;
        _elementVisual.CenterPoint = new Vector3(size.X / 2, size.Y / 2, 0);
        _elementVisual.StartAnimation("Scale", _springAnimation);
    }

    private void CreateOrUpdateSpringAnimation(float finalValue)
    {
        if (_springAnimation == null) _springAnimation = _compositor.CreateSpringVector3Animation();
        _springAnimation.FinalValue = new Vector3(finalValue);
    }
}

and add to your XAML

<Border BorderBrush="Red" BorderThickness="2" HorizontalAlignment="Center" VerticalAlignment="Center">
    <interactivity:Interaction.Behaviors>
        <appShell:ScaleBehavior/>
    </interactivity:Interaction.Behaviors>
    <TextBox Text="HELP"/>
</Border>

Solution with code in view model

<Page x:Class="UwpApp.AppShell.MainPage"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
      xmlns:local="using:UwpApp.AppShell"
      mc:Ignorable="d"
      Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
    <Page.DataContext>
        <local:ViewModel x:Name="ViewModel"/>
    </Page.DataContext>
    <Grid>
        <Border BorderBrush="Red" BorderThickness="2" HorizontalAlignment="Center" VerticalAlignment="Center"
                PointerEntered="{x:Bind ViewModel.PointerEntered}" PointerExited="{x:Bind ViewModel.PointerExited}">
            <TextBox Text="HELP"/>
        </Border>
    </Grid>
</Page>



public class ViewModel
{
    private readonly Compositor _compositor = Window.Current.Compositor;
    private SpringVector3NaturalMotionAnimation _springAnimation;

    public void PointerEntered(object sender, PointerRoutedEventArgs e)
    {
        CreateOrUpdateSpringAnimation(1.5f);
        Visual elementVisual = ElementCompositionPreview.GetElementVisual((UIElement) sender);
        Vector2 size = elementVisual.Size;
        elementVisual.CenterPoint = new Vector3(size.X / 2, size.Y / 2, 0);
        elementVisual.StartAnimation("Scale", _springAnimation);
    }

    public void PointerExited(object sender, PointerRoutedEventArgs e)
    {
        CreateOrUpdateSpringAnimation(1.0f);
        Visual elementVisual = ElementCompositionPreview.GetElementVisual((UIElement)sender);
        Vector2 size = elementVisual.Size;
        elementVisual.CenterPoint = new Vector3(size.X / 2, size.Y / 2, 0);
        elementVisual.StartAnimation("Scale", _springAnimation);
    }

    private void CreateOrUpdateSpringAnimation(float finalValue)
    {
        if (_springAnimation == null) _springAnimation = _compositor.CreateSpringVector3Animation();
        _springAnimation.FinalValue = new Vector3(finalValue);
    }
}