2000 Saturn SL2 sensor

Computer problem
2000 Saturn SL2 4 cyl Two Wheel Drive Automatic

I went to DEQ and he said "OBD 02Sensor says
unready. He asked me if Ijust got a new battery
or some work done. I said NO. Next day I
drive 51 miles to friends stayed 4 hours and
then drive 51 miles back to DEQ Same thing
unready. What is this please?
I live on SS only and do not have funds
for repairs. I need help with answer so
I can get this done to get my new tags
and get to my doctor appointment every
week. Thank you.