I just want to bump this since I’m having the exact same issue with the same version of Mac OS. I’ve opened an issue on github since now it is clear I’m not the only one experiencing this. I would like to know if there are any workarounds though. I’m happy to supply information to anyone willing to help.
What I know so far:
Serial communication does not work for both the Uno (OP) or the Mega (Me).
The Arduino does show up in the system report under USB.
The port does not show up when running “ls /dev/su*” in terminal.
It does not work using both a direct connecting with a USB B/C cable, nor does it work with a connecting to a USB A Hub, which itself is connected to the USB C port of the computer.
I have an AtMega16u2 chip to communicate between the usb and the microcontroller, so drivers shouldn’t be the issue (I don’t think, could be wrong about this)