If your OpenGB app interface appears with a red bar titled, "Error, not connected to the Raspberry Pi", here are a few potential troubleshooting scenarios to look out for:
1. Make sure that the black power toggle, located on the upper right side plate) for the printer is switched on.
2. Ensure that you have plugged in the printer's USB cord into the Pi to establish it's connection. The Raspberry Pi has 4 USB ports, which acts as a hub. All alike and any will work when connected to your OpenGB. The additional ports are open for plugging in your other devices (such as keyboard, mouse, etc). To mount the Pi, follow this guide. The location of the printer's USB connection is along the printer's top left side.
3. If you've checked all of your USB ports and connection issues to the Pi persist, you may be running an outdated version of the opengb package. This version did not have the step to add the opengb user to the "dialout" group. To do so, open the terminal (located on the Pi's top menu bar), and type the following commands:
sudo usermod -a -G dialout opengb