I just can't make this work. I am using Mac OS Mojave. I have the latest Evernote update. I just want to move a notebook into an existing stack. From the Notebooks page (the one headed My notebook list), I have tried dragging and dropping as I used to with a Windows computer; dragging and dropping while holding down the cmd key, and dragging and dropping while holding the ctrl key. Have also tried while holding down the alt key. Every time it's the same - the notebook won't move, and quite often it just opens. I am using the web-based version. Perhaps you can only do it with app? This was never a problem with a Windows computer, though.