Your response makes little sense. I got it working but it's not working.
So here are my thoughts. The I/O for USB is direct to the output selected by those switches with one caveat, if you route the USB out to main mix, the manual states it mutes USB recording to avoid feedback. If you are plugged into the phones output and are using a standard 1/4" headphone connector, the incoming signal from the computer is direct with no panning control, only level, and what is left and right is in the computer audio control panel or setup of the DAW I/O.
So if you are playing back computer audio and you have the USB send to Phones selected and send to Main off with headphones plugged into the Phone jack with the Phone knob turned up, and are still not getting audio, the issue is likely, in order, setting of computer audio out, faulty Phone output of mixer (test with another input on mixer) or possibly a bad adaptor for the headphones (1/8th inch to 1/4)
So first, does the headphones work with a microphone plugged in and can you pan audio left and right? If yes, issue is likely in computer. If not, it's the phone jack or headphones.