Xamarin Forms XAML OnPlatform VerticalOptions

What I'm trying to do may seem to be simple but I have been looking for around an hour with no luck so far.

I have the following XAML

<StackLayout Grid.Row="2" Margin="20,20,20,20">
    <StackLayout.VerticalOptions>
        <OnPlatform x:TypeArguments="x:String">
            <On Platform="Android" Value="CenterAndExpand" />
            <On Platform="iOS" Value="EndAndExpand" />
        </OnPlatform>
    </StackLayout.VerticalOptions>
</StackLayout>

But it's giving an error saying:

Cannot assign property VerticalOptions, property does not exists, or is not assignable, or mismatching type between value and property

I have done the same successfully but for other properties of type x:Boolean, x:Double and Thickness... Out of curiosity I've been also trying to find some documentation that outlines all the available x:TypeArguments as a reference but still with no luck as well.