What is the problem?
OctoPrint and the printer are operating completely normal and reliable but the under voltage warning is showing up. The printer and the PI are both running from a 800W PC Power Supply Unit (as the only user of that power), I doubt that it's a real issue.
What did you already try to solve it?
Ignoring the warning
Additional information about your setup
- Raspberry Pi 3 B+
- Printrbot Simple Metal
- PC Power-supply, PI connected to 5V, Printer to 12V