asp.net page with framework 2.0 andajax

I initially made an ASP.NET page and after I added
AJAX code by entering the page is the initial declaration

<% @ Register
Assembly = "System.Web.Extensions, Version = 1.0.61025.0, Culture = neutral,
PublicKeyToken = 31bf3856ad364e35 "
Namespace = "System.Web.UI" TagPrefix = "asp"%>

then I have write the ScriptManager in this page.

In the web.config page i have write the tag

<add assembly = "System.Web.Extensions,
Version = 1.0.61025.0, Culture = neutral,
PublicKeyToken = 31bf3856ad364e35 "/></ assemblies>.

Now when lunch the application gives me a problem SYS.

How can I solve this problem.

Thanks

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Prog.