Go to... Settings > Display > Screen timeout... select, 15, 30 sec's, 1, 2, 5, 10 min's
I had this same problem. I kept trying to change the screen timeout from the display settings but it would always say 10 mins no matter what I selected and turn off after 20 seconds. See if you have any apps that may affect the screen timeout. I had an app that I used call Kin screen to keep my screen on whenever I needed it to just stay on for an indefinite period of time. I didn't realize that it had a setting in there also to turn the screen off and it was set to 20 seconds. Made the change in the app and it fixed the problem.
THANK YOU!! this was driving me insane on my new s21 plus and it was kinscreen that was doing it! I owe you a drink, mate!