Noise Reduction process in Audition

I have a talking head video with a lot of background noise. 

I first selected both audio and video tracks and right-clicked to Unlink them. 

Then I selected the audio track and right-clicked to Edit in Auditon. 

In Audition > Effects menu > Noise Reduction > Noise Reduction process, I hit the Capture Noise Print button, but an error message comes up stating my clip is too short. 

When I follow the instructions and change the FFT to the smallest size option (512), it still doesn't work.  I have tried strining a few short background noise clips together, but it doesn't work.

Questions:

1) How long does a backgroung noise clip have to be?

2) What does FFT stand for, and why are the number options in that dropdown menu as they are?

3) How can I get this to work?