Declare the AnimationExtender with no TargetControlID in your .aspx. Then, in your Page_Load method, add AnimationExtender1.TargetControlID = Button1.UniqueID.
What happens is that your Button1 is becoming Wizard1_Button1 in the DOM of the page, so when the AnimationExtender tries to reference it with $get('Button1'), it can't find it. For whatever reason, setting it to Wizard1_Button declaratively doesn't work either. Setting it programmatically does work.