I get stereo track required while vocoding, but I already inserted one!
The vocoder works only on stereo tracks. To apply an effect to one or more specific tracks, select the track(s) that you want to apply the effect to.
See here for more information about the Vocoder: http://manual.audacityteam.org/man/vocoder.html
See here for more information about selecting tracks: http://manual.audacityteam.org/man/audacity_selection.html