Similar issue here -- I watched the sticky to set Settings USB Audio MIX, have not even opened my DAW yet, as soon as I set the song up, set the bit depth and sample rate, and suddenly I get the same error "UBS Fs mismatch" and sound is gone from the youtube sticky tutorial. Cannot figure out how to edit that song to change the bit depth/sample rate. So deleted the song. Still no sound from my mac which was previously coming back fine in ch 1 and 2. Guess I'll pull out the old laptop now, so I can start going through the youtube vids. Manual very very light on this error.
EDIT -- I think I solved my issue. In MacOS, was able to launch AUDIO MIDI SETUP, and that is where I found the 44.1 setting in OUTPUT tab - switched to match what I had in SONG, and seems to be ok now (I got sound back, from my mac, back into track 1 and 2).