So you’ve got a serial device that needs printing output sent to it and you’ve installed Ubuntu 11.04. Cups responds with an Unable to open device file … Permission Denied. Well!
Ubuntu has thoughtfully explicitly denied access to /dev/ttyUSB devices through apparmor. No reason you’d ever print to serial devices in Ubuntu, eh?
So to get your receipt printer, Okidata 320ML serial printer, or whatever working you’ll either need to uninstall apparmor (I imagine) or change a line in the file /etc/apparmor.d/usr.sbin.cupsd from “deny /dev/ttyUSB*” to “/dev/ttyUSB*”.
That should do it.