Phil,
Someone gave you this command, but they forgot to start it with "sudo" and if you didn't use sudo the command will not work.
sudo chmod 777 /dev/ttyUSB0
Check the ownership of this:
ls -l /dev/ttyUSB0
This will list the owners of /dev/ttyUSB0.
See if you're in the group that is listed.
Enter this command:
groups
It will return the groups your a member of. You have to belong to the group that is listed when you gave the command: ls -l /dev/ttyUSB0
If you disconnect the USB cable then reconnect it. You can confirm the device name by this command:
sudo dmesg | tail
You should see lines that says "new device" and it will give the address of the new device, that's the device CHIRP needs to use in order to work .
Try that,
73