What wrong with the code?

You passed NULL instead of a valid handle as the uIdNewItem parameter.

From nf-winuser-appendmenua

uIDNewItem

Type: UINT_PTR

The identifier of the new menu item or, if the uFlags parameter is set to MF_POPUP, a handle to the drop-down menu or submenu.

In a Windows desktop application a handler was added for WM_CREATE to create and add menu items to the menubar when the main window is created-

Handler -

case WM_CREATE:  
{  
	HMENU hMenuBar = GetMenu(hWnd);  
  
	// Create popup menu with code  
  
	HMENU hPopup = CreatePopupMenu();  
	AppendMenu(hPopup, MF_STRING, ID_DYNAMIC_ITEM1, _T("Code created Item 1"));  
	AppendMenu(hPopup, MF_STRING, ID_DYNAMIC_ITEM2, _T("Code created Item 2"));  
	AppendMenu(hMenuBar, MF_POPUP, reinterpret_cast<UINT_PTR>(hPopup), _T("Code Created"));  
	DestroyMenu(hPopup);  
  
	// Get popup menu from a menu resource  
  
	HMENU hDummy = LoadMenu(NULL, MAKEINTRESOURCE(IDR_MENU1));  
	AppendMenu(hMenuBar, MF_POPUP, reinterpret_cast<UINT_PTR>(GetSubMenu(hDummy, 0)), _T("Menu Resource"));  
	DestroyMenu(hDummy);  
}  
break;  
  

Menu resource used -

Running application with added menu items -