XAML Ellipse not working - xaml

Question

I can't seem to get Ellipse to work at all in my Xamarin project.
It just says "The type or namespace name 'ImageBrush' does not exist in the namespace 'Xamarin.Forms' (are you missing an assembly reference?)" and I have no clue what's up. I've checked video's but there everything seems to work fine...
Anyone know what I'm doing wrong?
EDIT:
I forgot to post the code along with the question. Thanks for the reminder!
I'm trying to get the Logo.png to display as a circle. I
I can't seem to get Ellipse to work at all in my Xamarin project.
It just says "The type or namespace name 'ImageBrush' does not exist in the namespace 'Xamarin.Forms' (are you missing an assembly reference?)" and I have no clue what's up. I've checked video's but there everything seems to work fine...
Anyone know what I'm doing wrong?
EDIT:
I forgot to post the code along with the question. Thanks for the reminder!
I'm trying to get the Logo.png to display as a circle. I
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="GrowersClassified.Views.LoginPage">
<StackLayout x:Name="MasterLayout">
<StackLayout x:Name="LogoStack" Padding="40, 80, 40 , 0">
<Ellipse Height="100" Width="100">
<Ellipse.Fill>
<ImageBrush ImageSource="Logo.png"/>
</Ellipse.Fill>
</Ellipse>
</StackLayout>
<StackLayout Padding="40, 20, 40, 0">
<StackLayout x:Name="LoginEntries" VerticalOptions="StartAndExpand">
<ActivityIndicator x:Name="ActivitySpinner" Color="Red" IsRunning="True"></ActivityIndicator>
<Label x:Name="Lbl_Username" Text="Username"/>
<Entry x:Name="Entry_Username" Placeholder="Username"/>
<Label x:Name="Lbl_Password" Text="Password"/>
<Entry x:Name="Entry_Password" Placeholder="Password"/>
<Button x:Name="Btn_Signin" Text="Log in" Clicked="SigninProcedure"/>
</StackLayout>
</StackLayout>
</StackLayout>
</ContentPage>