For those of you interested in this post, I have some more information - the lack of AutoComplete seems to only be an issue with the desktop editor itself and with Google Chrome (which is a bit odd since the Sense desktop editor is based on Chromium).
To get around the issue you can use one of the other browsers: if you are using the desktop hub you can do this by opening the desktop first to get the hub running, and then open an alternate browser and going to the URL localhost:4848/hub
Hope this helps
I had the same issue and after playing with a bunch of chrome://flags settings to no avail, I discovered a work around.
Once you open the expression editor in Chrome just press Shift+Tab once... for whatever reason that seems to get not only the auto-complete working, but also inserts tabs into the text as opposed to switching focus to another control.