I have just updated my MacBook Air to High Sierra but weirdly I cannot adjust the brightness with the keys on the keyboard.
Anybody have an idea why this is happening and how to fix?
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Are they set to be function keys - there is the "fn" key - have you tried holding that down and tapping the lighting key?
If that works, then you can change the behaviour in system preferences.
You can check this functionality in System Preferences > Keyboard > Keyboard Tab and check to see if the bottom check box ("Use F1, F2, etc...") is checked