To animate thickness, use a Storyboard like this (from msdn example):
<BeginStoryboard>
<Storyboard>
<!-- BorderThickness animates from left=1, right=1, top=1, and bottom=1 to
left=28, right=28, top=14, and bottom=14 over one second. -->
<ThicknessAnimation
Storyboard.TargetProperty="BorderThickness"
Duration="0:0:1.5" FillBehavior="HoldEnd" From="1,1,1,1" To="28,14,28,14" />
</Storyboard>
</BeginStoryboard>
Actually, to animate any property that takes values as "w,x,y,z" you use a ThicknessAnimation
It seems to me that what you want to do is move the red rectangle to the right.
In that case, put the whole thing in a Canvas
and use a DoubleAnimation on the red rectangle's position.
Either way, the error you're getting does not come from the small piece of code you provided, if you want to adress that, please provide use with more code.
Edit: since ThicknessAnimation seems to be not available on WP7, try this instead:
<BeginStoryboard>
<Storyboard>
<DoubleAnimation
Storyboard.TargetProperty="BorderThickness.Top"
Duration="0:0:1.5" To="15" />
<DoubleAnimation
Storyboard.TargetProperty="BorderThickness.Left"
Duration="0:0:1.5" To="25" />
</Storyboard>
</BeginStoryboard>