I had a similar problem a long time ago when hooking up deck to Xpress pro. This is a shot in the dark and I hope it helps. This was on PC not a Mac
I kept getting this message “the port on Com1 is busy and cannot be used for deck control.”
Avid support center fix:
“The most likely reascon is that the port has been chosen for use in the CONTROLLER SETTINGS.
To fix, double click the CONTROLLER SETTINGS, from the SETTINGS tabin the PROJECT WINDOW.
If either PORT or GAIn CONTROLLER PORT are set to COM1 change them to
NO PORT