This was SOOO frustrating but I finally figured it out with the clue in the text "...click on each liquid engine..."
Too bad I didn't know the terminology anywhere well enough by this stage to know which type of engine is which. I had to read this thread a couple of times before I realised the relevant parts.
I *WISH* the action items for each step of the tutorials were bullet pointed and used exact names. Reading through all that stuff might add flavour but it's a pain in the neck! Especially with the text sized the way it is (yes, I have the UI scaled to 150% but I'm OLD and I can't see, no, not even with my good glasses).