Cannot adjust brightness after Big Sur update

angeles212 Said:

"Cannot adjust brightness after Big Sur update: Hello, ever since I upgraded to Big Sur the toolbar brightness keys and the keyboard light do not work on my MacBook Pro 2017.[...]"

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Reinstalling the macOS:

One option would be to reinstall the macOS. Perhaps the install was not 100% successful? Sometimes, when major changes are made to your Mac(i.e. update installations), system settings (i.e. for Brightness) become manipulated, technically. So, when reinstalling, nothing would be deleted. It's just that items that have become corrupted or deleted, would be replaced or reinstalled back to where they should be. Go Here: How to Reinstall macOS from macOS Recovery - Apple Support

Important: First, Back Up your Mac:

Create a Time Machine backup of your Mac, so that you can have something to restore your Mac from, should anything go wrong with the reinstall. Take this as a learning experience, knowing to always create a backup of your Mac, prior to making any major changes to it (i.e. update installation)

In your [System Preferences] > Display, do you have [Automatically adjust brightness]? If so, disable it.

I'm using Catalina and had the same problem after security update. After I disabled [Automatically adjust brightness], I can not adjust display brightness with my keyboard keys.