Section of Code Causes Ajax Not to Display

<!-- ================================================================================ --> <!-- SECTION 6: Error Message Display --> <!-- ================================================================================ --> <asp:Button ID="BtnErrorPopup" runat="server" Text="" CssClass="btn-modal" /> <ajx:ModalPopupExtender ID="MdlErrorPopup" runat="server" BackgroundCssClass="bg-modal" TargetControlID="BtnErrorPopup" PopupControlID="PnlErrorMessage" OkControlID="BtnErrorMessageClose" /> <asp:Panel ID="PnlErrorMessage" runat="server" CssClass="error-message" Style="display: none"> <asp:Label ID="ErrorHeading" runat="server" Text="" CssClass="head" /> <div class="content"> ...