Reaktor 6 is not showing up as an Instrument, although other NI Instruments do appear (Kontakt, Massive X, et.). The VST3 is installed in the default MacOS location. I rebooted, restarted Live, and I rescaned the plugins. I found a workaround by setting the Instrument to “No Instrument” and adding “Reaktor 6 FX” as an effect plugin on the track, but that is as pretty terrible solution.
(Scaler 3 v1.0.3 VST3, Ableton Live 12.1.11, MacOS 15.3.2)