In this new window click “Display” to open up the listings under it.Click the “Change plan settings” link next to whichever plan you are currently using and then click “ Change advanced power settings“.
Restart windows and check This will fix “can’t adjust brightness ” bug.
You Can check And install latest updates from Settings ( Windows + I ) -> Update & Security -> windows update -> And check for updates. If the problem started after install windows 10 version 21H1, then make sure you have installed the latest Cumulative Update, As Microsoft mention: “ Addresses an issue with the brightness controls on some laptops after updating to the Windows Update.
