Hi danlevy,
If your command prompt is giving you errors I’d suggest you to install another terminal emulator such as minicom. After you’ve installed it, manually edit your path with the command sudo nano/private/etc/paths. Then add /opt/minicom/2.2/bin to the bottom of the file and save. Run minicom -s first to configure your serial interface device name, and other options. Your serial interface device name is found with ls /dev/cu.* After doing this try to run your screen command once again. Let me know if this works.
Regards
Sergio