The function keys in FL Studio open various types of windows, like the Mixer, Playlist etc. On a Mac they work by pressing down the 'fn' key simultaneously with the desired function key (F1, F2 etc.). If you wish for the keys to work on their own without having to press 'fn', you can navigate to System Settings > Keyboard > Keyboard Shortcuts > Function Keys and turn on the option 'Use F1, F2, etc. keys as standard function keys'.
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