Hello Marcos,
In an open Terminal window at the command prompt, try typing
. The results should return all possible usbmodems available for communication.
Hopefully, the 'ls' command returns something like: tty.usbmodem1412
(Note: the number will differ)
If one usbmodem returned from the ls command, try typing
where the number will match what was returned by the 'ls' command.
Hopefully, that will help. If you receive the 'terminating' message, that particular usb communication port is in use. My recommendation would be to turn off the system. Unplug the micropython board. Boot the mac. After it is booted, plug in the micropython board. Open a terminal. Try the 'ls' command and screen command again.
Good luck,
Carol