How to fix "Parser Error Message: Unknown server tag 'asp:ScriptManager'"

Hi Guys.
when upload files on Server Host, i receive this Error:
Server Error in '/' Application. Parser Error Description: An error occurred during the parsing of a resource required to service this request. Please review the following specific parse error details and modify your source file appropriately. Parser Error Message: Unknown server tag 'asp:ScriptManager'. Source Error: Line 10: <form id="form1" runat="server"> Line 11: <div> Line 12: <asp:ScriptManager ID="ScriptManager1" runat="server"> Line 13: </asp:ScriptManager> Line 14: <asp:UpdatePanel ID="UpdatePanel1" runat="server"> Source File: /Default.aspx Line: 12 Version Information: Microsoft .NET Framework Version:2.0.50727.5483; ASP.NET Version:2.0.50727.5483
default.aspx:
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