Hi, there is a solution that worked for me (M2 pro max 16" 2023) and that was found in [link|https://discussions.apple.com/thread/255107729?sortBy=rank|apple forum here]:
[quote|format=featured]Solution found: I had changed the colour profile to In ternet & Web (sRGB). As soon as I changed it back to Apple XDR Display, the settings unlocked and the brightness can be adjusted. Case closed :-)
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I was really hopeless after reading previous answer!