noise reduction flatlines all

I am using Windows 10 and Audacity 2.3.3
When I use the Noise Reduction feature, I get a sample, test it; when I apply it to the whole recording, it flatlines - no intelligible sound at all.
Thank you for your help!

Noise reduction works in two steps. Select some of the pure noise—the exact noise you don’t want.

This can be a problem if the performer never shuts up long enough to give you a shot at the noise by itself. Noise Reduction doesn’t work without this step.

Effect > Noise Reduction > Profile.

Then select the whole show or performance with the select button on the left and apply the reduction.

Effect > Noise Reduction.

Gentle audiobook reduction can be 6, 6, 6, up to 12, 6, 6. Or use the default values in the instructions. Then press OK.

To high a Noise Reduction will make your voice sound like a bad cellphone. You have to get basic sound recording down before you try to fix problems with the Audacity tools.