Thanks for reporting. I think that comment might be misplaced - it looks like in this case, REAPER is trying to enable all 64 possible speakers, but there’s no way of representing that layout in JUCE at the moment.
Either this case shouldn’t assert, or the warning should be moved to setBusArrangements in the plugin client code, to warn that the host has requested a layout that JUCE can’t use. I’ll have a think and update this thread once we’ve pushed a fix.