If using a USB serial converter, as many 3d printers seem to, we can find the device here:
ls -lah /dev/ttyUSB0
crw-rw---- 1 root dialout 188, 0 Aug 27 15:41 /dev/ttyUSB0
As you can see, the device is owned by root and in the group dialout.
In order to allow ourselves, or our Octoprint service account, access we need to add that user to the dialout group.
sudo usermod -a -G dialout $USER
How do we know the command worked? One way is to check the group list:
sudo groupmems --group dialout --list
portaj # or whatevr the value of your shell's $USER is at the time you ran the command above.
If there is a current session for the user you added then you may need to logout and login again for it to take effect. Or, you can use:
newgrp dialout