Hi, I've hit a surprise snag right at the start with this lesson.
I have the sapling addon enabled (also restarted Blender just in case) and I'm using Blender 3.1 but when I add a tree, I can't edit any of the options - they are greyed out and it's game over.
I'll add a screenshot (this window won't let me)......
Ok, couldn't add an image (error 401) BUT I have found a solution which I'll leave here in case somebody else gets the same issue.
Apparently you need to have "global undo" enabled, which is now under Preferences-System.
Thanks Smartassmusic
Global Undo is on by default and the name and Tooltip aren't very clear about it, but Undo and Repeat Last Operation don't work at all when you disable it (and I don't know what other desasters it might cause).