Well, on this Doc page, they say you should open Settings, then click Editor and find General:
When going to my Settings, this is what I see and I see no General, Legacy editor or Live Preview setting:
Here, I can read that you seem to need to pay for the Catalyst license of Obsidian to get Live Preview, hence my question @Dor: Do you have a free account or do you have a paid one?
That may explain why I can’t find the Legacy Editor and the Live Preview?