Can you give details about exactly what commands you used to install opam, and what version of opam? Did you run opam init to do the initialisation? Did you run opam update to download the latest package metadata?
Ah. Thanks! The problem must have been that I had some ancient opam stuff hanging around from when I played with it a long time ago, and needed to do opam update and opam upgrade. I’m good to go now.
Seems like it might be worth adding something to the tutorial saying to do that if there’s a problem with running the given opam install, but maybe there are too few people with this problem for it to be worth it.