What is the meaning of the following error message?How can I use the EnableClienTValidation()?

Error 3 'System.Web.Mvc.HtmlHelper' does not contain a definition for 'EnableClientValidation' and no extension method 'EnableClientValidation' accepting a first argument of type 'System.Web.Mvc.HtmlHelper' could be found (are you missing a using directive or an assembly reference?) c:\Dev\DEV\test3\Code\MvcUI\Views\Customer\Create.aspx 11 13 MvcUI

I have reference the following:`" type="text/javascript">

    <script src="<%=Url.Content("~/Scripts/jquery.validate.js")%>" type="text/javascript"></script>

    <script src="<%= Url.Content("~/Scripts/MicrosoftAjax.js")%>" type="text/javascript"></script>
    <script src="<%= Url.Content("~/Scripts/MicrosoftMvcAjax.js")%>" type="text/javascript"></script>
    <script src="<%= Url.Content("~/Scripts/MicrosoftMvcJQueryValidation.js" )%>" type="text/javascript"></script>

`