Thanks to DTLow, I realized that the sidebar is collapsible. To create a new stack or add to a stack in this version of Evernote on MacOS 10.15, you must first collapse the sidebar and work within the 'Notebooks' page.
Creating a stack still DOES NOT WORK from the sidebar. I've never realized that there -is- a separate 'Notebooks' page in the desktop client, since it simply replicates what is visible in the sidebar list of notebooks. It does center the panel, though, so I guess that's a win. it obviously DID work in previous versions of Evernote, since that's where I always created stacks, until it stopped working.