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.
Comments

For me it entered as "Function"
Weird. For me, just pressing fn
does nothing, but fn+F8
or something sets it as LMETA
.
I'm using a macbook, for me it also enters Function.
Is this still an issue in the latest snapshot 16w44a? If so please update the affected versions.
This is an automated comment on any open or reopened issue with out-of-date affected versions.
I'm using a MacBook Pro with OS X Sierra and it isn't a fix but I was able to rebind mine by holding the function key, then clicking the button I wanted to remap, and while still holding the function key pressing the desired f# key. it worked for rebinding with me at least. As I have said though not a solution but a workaround to achieve the desired result,

This is fixed for me in 17w43a. If someone is still experiencing this issue I will reopen.
Confirmed. For some reason, just pressing the
fn
key does nothing.