Probably, the message you are receiving is because you are exporting with 2 track ( a stereo pair ) of audio. Try exporting with 8 tracks of audio, even if you need to disable the last 6 tracks. But be sure to have 8 tracks in your timeline.
I was receiving the same error message using Final Cut Pro 7, and there, I had to change the audio settings of the sequence to 8 output channel, dual mono each. And then I could export fine.