This message is telling you that you are trying to run two Liquid Rhythm plugins at the same time in your current DAW session.
This error will only occur with Liquid Rhythm 1.4.5 (and earlier), and with early versions of Liquid Music. Liquid Music 1.8.0 + Liquid Rhythm 1.8.0, as well as Liquid Music 1.7.0 + Liquid Rhythm 1.7.0, allow multiple instances of Liquid Rhythm VST / AU in the same session.
Please note that the Liquid Rhythm VST / AU plugins can still not be run simultaneously with the Liquid Rhythm / Liquid Music Max For Live plugin; you must choose one or the other, and restart Ableton Live if you would like to switch between Max For Live and VST / AU plugin types.