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: https://www.audacityteam.org/download/mac/)
The problem was eventually resolved, but unfortunately they were not sure which step fixed it. You can find the topic here: https://forum.audacityteam.org/t/cant-record-from-usb-interface-after-upgrade-to-mojave/50762/1