Problem is you're trying to use something from an assembly which is not referenced. you need to add that in the Window/Page tag at the root with other namespaces as
xmlns:vsm="clr-namespace:System.Windows;assembly=System.Windows"
Then you can use it as
<vsm:VisualStateManager.VisualStateGroups>
<vsm:VisualStateGroup x:Name="CommonStates">
<vsm:VisualState x:Name="MouseOver">
<Storyboard>
<ColorAnimation Storyboard.TargetName="tickBox"
Storyboard.TargetProperty="(Rectangle.Fill).
(SolidColorBrush.Color)"
To="PaleGreen" Duration="0:0:0.5" />
</Storyboard>
</vsm:VisualState>
</vsm:VisualStateGroup>
</vsm:VisualStateManager.VisualStateGroups>