Press the Setttings button (the cogwheel in top right corner), switch to the Document tab, there should be both Categories and Tags.
Thanks, but afraid not (screenshot)
Do you see any errors in console? (if using Chrome, press F12 and switch to Console tab)
Also check this:
https://github.com/WordPress/gutenberg/issues/8763
If you have iThemes Security, it could be the one causing this…
Bingo, iTSec it is!
Thanks for bringing it to my attention, very much appreciated.
Th