Actually is a Chan_Dongle issue not asterisk not freepbx.
The error is telling you that Chan_Dongle is already loaded in your asterisk, and the secod is telling you that the interface on the linux system doesn't exist. So first of all check which device is using linux to configure the USB-CDC by running:
ls -lha /dev/tty*
Or unplug the usb modem run the command
tail -f /var/log/messages
attach again the usb and look in the log which tty is assigned to it.
Then go and edit the /etc/asterisk/dongle.conf file accordingly. Save changes, go back to the cli, unload the module, load the module and try again.