If you’re looking to install the new Inform 10.1 (currently in beta), see Beta release of Inform App for Linux. This should install without issues on an Ubuntu 20.04 derivative.
If you’re looking to install the previous release of Inform 7 6M62, you may have luck with the thread at Gnome-inform7 6L38/6M62 IDE now running on modern Ubuntu & Fedora OS. Post #74 has some instructions that may be useful to you; you’ll need to dig up older versions of certain packages online.
Thank you very much! Both seemed to work, although installing the beta seems to have replaced the older version rather than running alongside it as I expected. Would you recommend one of the versions over the other? Do either have outstanding problems or advantages over the other?
I seem to have answered my question for myself — for anybody else wondering, the beta version was having problems for me, but so far the older version is working fine. Crossing my fingers that it continues to work!