When using a Mac (or many laptops) with default keyboard settings, which require the FN+F# to perform F#, you cannot rebind to F# because it interprets the FN as the key press, rather than the F# that results in pressing one of the F# keys while holding FN.

Workaround is to change your keyboard settings to require FN+F# to perform things like volume, brightness, media controls, and have the keys perform their respective F# function by default.