... a msg saying 'Cannot open /dev/cuaU0'.
Very likely permission problem. Try the same operation as root. If it works, check the ownership and permissions of /dev/cuaUxx. If it is wrong, then it gets gnarly. As a quick hack, you could just do a
chmod a+rw /dev/cuaU*
. Two problems with this approach: First, it is insecure, now everyone logged into the machine can use this device. But if you know that you are the only person to ever log in, this may not matter. Slightly better variant is to change the ownership of the device to a different user or different group, but that requires more thinking and typing. Second, the /dev/cuaUxx device will vanish if you unplug the USB device, and be recreated when you plug it back in, and the new device (which will have the same name!) will have forgotten that you changed the permissions (or ownership).
There is a clean solution: Somewhere there is a config file that specifies the desired ownership and permissions when new devices are created.
I think in FreeBSD this is handled separately for USB devices, but I'm not sure, and I don't have time this morning to do the hunting and gathering to find out. I had to do this a few months ago on Linux with systemd for work, and there it worked well and quickly, because the required config file was easy to find. Please do some google searches, and search this forum and the FreeBSD handbook, and you'll probably find it pretty quickly yourself.
EDIT: See aragats' post below, with the correct answer.