Hi Nishant,
Remove following line of code from your programe:-
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
And Add following:-
<asp:toolkitscriptmanager id="ToolkitScriptManager1" runat="server" xmlns:asp="#unknown">
</asp:toolkitscriptmanager>
Note:- Whenever you are using Extender controls then you should use toolkitscriptmanager instead of Scriptmanager.
Also do the following If after applying above you are still getting any error:-
Go to Tools> NuGet Package manager > Package Manager Console
write
PM> Install-Package AjaxMin
Press Enter.
Thanks
Mukesh