Situation

在.aspx 宣告 telerik:RadAjaxManager 前未先建立 telerik:RadScriptManager
Source Code 如下

    <!--RadScriptManager應該在RadAjaxManager前建立
    <telerik:RadScriptManager ID="RadScriptManager_Main" runat="server">
    </telerik:RadScriptManager>
    -->
    <telerik:RadAjaxManager ID="RadAjaxManager1" runat="server">
        <AjaxSettings>
            <telerik:AjaxSetting AjaxControlID="RadButton1">
                <UpdatedControls>
                    <telerik:AjaxUpdatedControl ControlID="RadLabel1" />
                </UpdatedControls>
            </telerik:AjaxSetting>
        </AjaxSettings>
    </telerik:RadAjaxManager>

Action

在建立telerik:RadAjaxManager之前先建立telerik:RadScriptManager

    <telerik:RadScriptManager ID="RadScriptManager_Main" runat="server">
    </telerik:RadScriptManager>
    <telerik:RadAjaxManager ID="RadAjaxManager1" runat="server">
        <AjaxSettings>
            <telerik:AjaxSetting AjaxControlID="RadButton1">
                <UpdatedControls>
                    <telerik:AjaxUpdatedControl ControlID="RadLabel1" />
                </UpdatedControls>
            </telerik:AjaxSetting>
        </AjaxSettings>
    </telerik:RadAjaxManager>

Result

不再出現
The control with ID ‘RadAjaxManager1’ requires a ScriptManager on the page. The ScriptManager must appear before any controls that need it.