If you are not running Arduino as root there is chance you’ll see this error message:
can’t open device `/dev/ttyUSB0’: Permission denied
If we look at the device in question, we can see that it’s owned by the dialout group.
$ ls -al /dev/ttyUSB*
crw-rw----. 1 root dialout 188, 0 nov. 23 20:49 /dev/ttyUSB0
crw-rw----. 1 root dialout 188, 1 nov. 23 20:50 /dev/ttyUSB1
So to fix this you just have to add yourself to the dialout group and log in again.
$ sudo usermod -aG dialout $USER