No Audio in Garageband from USB Mixer

I have a USB Mixer (Mackie ProFX16) and I am trying to set it up to use on my iMac. It's a 2011 iMac running Sierra (v. 10.12.3) and I'm trying to use the mixer with Garageband (v. 10.1.6).

I have gone into system preferences and have selected "USB Audio CODEC" but no sound. When I open Garageband, I select "Empty Project" and where it tell me to select the audio input I save selected "USB Audio CODEC"

When I create a new track I select the Microphone "Record using a Microphone or line input"

But when I press record, nothing comes through. I know there is nothing wrong with the mixer because I tested it with my PC in audacity.

What am I doing wrong?