You have not told to what item/command you were trying to assign your shortcut, but – in general – Illustrator has got pretty old-fogyish rules when it comes to custom shortcuts. Per se rules which cannot be explained and excused reasonably.
The Control Key in Illustrator cannot be used on the Mac as a keyboard shortcut.
The reason is that the Adobe team or at least the old Adobe team is convince that it will interfere with the shortcuts provided by the system.
Even though it is pointed out that those shortcuts cn be disabled.
Unless things have changed you can argue till you are blue in the face but you will not get the Team to reconsider inplementing the use of the control key in Illustrator for keyboard shortcuts.
Try it you'll see what I mean Iargued this when AI 9 was out. Igot respnses from Adobe but all I got for the responses were arguments
I just love it when a good idea is passed over because "that's the way we've always done it." :-/ Time for some new thinking in that "team" then.
Turning off system keyboard shortcuts is ESSENTIAL with MANY applications (especially in the 3D world), and so it should not be a problem for Ai either.
But oh well.
Oh well here too!