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.