Audacity Not Recognising Mixer

There was a very similar problem posted a while back. They were also using a USB device, Audacity 2.3.0 and Mojave. (You have not said, but I assume that you are using Audacity 2.3.0 - if not, you can update from:

The problem was eventually resolved, but unfortunately they were not sure which step fixed it. You can find the topic here: