I'm running v7.14 under Catalina. In my desktop app, I do not have the ability to move a new notebook to an existing stack. All the Google entries plus the Evernote help here on their site says the "more options" ellipsis in the new notebook should have a "move to stack" option and I don't have that (see screenshot). I don't know why that first option is greyed out. Right clicking on the notebook in the sidebar doesn't have it and dragging and dropping into the stack doesn't work.