Start a new sheet, and just test the formula with the default sheet names to be completely sure.
I think this is it, based on the link below. My take is that it’s okay if the file is on Google Drive but not if you’re using Google Sheets to edit. I don’t have the means to test this out, though. Good luck!
https://support.google.com/docs/thread/131535458/error-unknown-range-name?hl=en
Folks, we have a winner!
As soon as I downloaded it to my desktop, it worked.
Thanks, everyone!