I am not sure about the error you get. I guess you have added the ScriptManager control on the page and then you are getting the following error:
Error message "Microsoft JScript runtime error: 'Sys' is undefined"
This error occurs when you have added a control from RadControls for
ASP.NET AJAX but your application is not configured to use ASP.NET
AJAX. To fix it you need to follow the
steps described at http://ajax.asp.net/docs/ConfiguringASPNETAJAX.aspx
(check the topic called
Adding ASP.NET AJAX Configuration Elements to an Existing Web Site