I had a similar problem and I attempted a number of different solutions. Just wanted to share what made it work for me. Problem Tried to use the screen program to connect my lab PC to a Raspberry PI via the USB ports and an RS232 cable. Ran the command screen /dev/ttyUSB0 Got the following error messages in quick succession. “Cannot access line ‘/dev/ttyUSB0 for r/w: permission denied” “Couldn’t f