That error in because you are working inside a page that inherits from a
Master and you have two
ScriptManager declared.
For sure you have and ScriptManager inside the
Site.Master and you are algo adding a
ScriptManager in this page
(Default.aspx) too.
Remove the scriptManager inside the childPage, because this page is going to use the
Site.master scriptmanager.
Remove:
<asp:scriptmanager runat="server" id="ScriptManager1" enablepartialrendering="true" xmlns:asp="#unknown">
</asp:scriptmanager>