I think Steve is on the right track when he mentions checking that your track is being sent to the mixer correctly...I notice on your screen shot that you have the routing set as "USB Audio Device". Normally for mixing this would be set to "Master" (or, in a more complex mix, to a Bus that then goes to the Master).
If you're feeding your track direct to the USB interface not via the mixer, then Audition can't mix it and export it!