UpdraftPlus says that a backup to Dropbox completed, but I don’t yet see it in my Dropbox

Are you looking on the Dropbox folder on your computer, and not yet seeing your backup?

This almost certainly means that the Dropbox folder on your computer is not yet synced with your Dropbox account. This can take a while, especially if your backup is big. Instead of waiting for your computer to sync with Dropbox, just go and view your Dropbox directly – go to www.dropbox.com, and view it there (look in Apps/UpdraftPlus).