Last update January 31, 2023 / This article concerns Mac OS X users only
MIDI Learn function works via MIDI protocol (MIDI CC message), thus requires effect to be able to receive MIDI messages (meaning having active MIDI input).
It is possible to take advantage of that feature with our Audio Unit effect plugins, but requires some extra steps to set it up properly (requirements of AU MIDI Controlled Effect specification). You should check out this entry at our FAQ to learn how to setup an AU MIDI Controlled Effect in Logic to be able to use AU's MIDI input (thus MIDI Learn).
Naturally this problem doesn't concern AU instruments, since they have MIDI inputs by definition (necessary for receiving MIDI notes).