I bought a new 3D printer yesterday, and am in the process of setting it up.
A ROBO3D R1 printer (purchased second-hand)
Desktop computer running Ubuntu 18.04
As per the manufacturer recommendations, I downloaded MatterControl v1.7 as the software used for actually printing the files (specifically, converting the .gcode and preparing slice files). This software wasn't able to connect to my printer, running on /dev/ttyACM0.
Turns out that the user running my print job didn't have file privileges to connect to this port. This was dumb and I should have thought of it earlier. But I didn't, so instead I'm writing up the steps to make the connection work here:
sudo usermod -a -G dialout robert
Then, just remember to log out and log back in as your user to allow the change to take effect. I strongly advise against running the software as root. This is just as easy, and protects you from nasty security issues.