If the sensor detects the paper that remains in the tray incorrectly, Paper Low is displayed even if more than half of the paper is loaded in the paper tray.
Remove all the paper from the paper tray and then reinsert it back in the paper tray.

Slowly push the paper tray completely into the machine.
DO NOT push the paper tray into the machine forcefully or quickly.
Doing this may cause the sensor that detects the paper remaining to operate incorrectly.