I was trying to catch up on this while I was in the hospital so I’m going to blame this question on the medication. Yes, that’s it, the medication. Thank you for understanding.
A few people have found this tricky, so I’ve made it a bit simpler.
You scroll to the bottom of the course unit and click on the Mark Complete button – which was pretty nondescript. I’ve just changed the formatting on it to make it a bit more obvious: