Hi A.Samarian,
That property is only available on the AJAX version of Script Manager so most likely you have something wrong in your Web.config.
In the Web.config, script manager is mapped to the MS AJAX (System.Web.Extensions.dll). Make sure your web.config has this:
<httpHandlers>
<!-- AJAX Extensions support -->
<remove verb="*" path="*.asmx"/>
<add verb="*" path="*.asmx" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
<add verb="*" path="*_AppService.axd" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
<add verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" validate="false"/>
<add verb="GET,HEAD,POST" path="*.asbx" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" validate="false"/>
<add verb="*" path="*CaptchaImage.ashx" type="Subkismet.Captcha.CaptchaImageHandler, Subkismet" />
<add verb="*" path="*.mml.ashx" type="mojoPortal.Web.Services.MathMLHandler, mojoPortal.Web" />
</httpHandlers>
<httpModules>
<!-- AJAX Extensions support -->
<add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
</httpModules>
Make sure those things are not missing from your Web.config
Hope it helps,
Joe