AfterVentura update, "Show sidebar" in Finder does not WORK!

I´m assuming you want to see the sidebar on an open Finder window...? With a Finder window open go to Finder > View (not Finder preferences!) then look down the open menu to "Show sidebar". You can also toggle the sidebar on and off as you like by using Control + Command + S on your keyboard. Hope this helps you!

Usr wrote " Checking "Show sidebar" in Finder prefs HAS NO EFFECT! NOTHING HAPPENS

That would because one needs to look in Finder >> Setting >> Sidebar