You are correct, I am working in the sidebar. How do I get to the Notebook panel? Sorry, I am being particularly dense this morning and the Help system comes up empty when I search for "notebook panel".
I am unable to drag old notebooks into an archive stack. This functionality suddenly stopped working a month or so ago. I am running Evernote v7.9 under macOS Mojave 10.14.3 (i.e., I am totally up to date) on a late 2015 iMac. I have about 35 current notebooks and about the same number in the archive stack.