I get that message, but only with the VST3 5.1 Premium Air, and only if I insert it with a Stereo montage, or with a 5.1 montage before the Master Section is initialized 5.1 (before playing a 5.1 montage). So basically it works as I would expect in Wavelab, since the Master Section only turns into multichannel when you play a multichannel montage, I would expect to need to do that before inserting the plugin.
But strangely here, the VST2 5.1 Air doesn’t seem to care whether the montage and master section are Stereo or 5.1, I get no error messages at all with the VST2 5.1 Air. (but my VST2 5.1 is not called “Premium”, only the VST3 is.)
Testing this on Windows.