DIAGNOSIS The paper length sensor in the paper tray is dirty or the paper stack is misaligned. Dust or a small piece of torn paper can block the sensor.
FIX Remove all paper from the tray. Use a can of compressed air to blow into the empty tray, focusing on the small rectangular sensor window at the back. Wipe the metal paper length guide with a dry cloth. Reload paper, making sure the stack is under the fill line and the guides are snug against the paper edges.
IF THAT DIDN'T WORK The paper pickup roller inside the printer is worn or dusty. Clean it with a lint-free cloth dampened with distilled water. You'll need to open the rear access door to see the rubber roller.
HEADS UP Do not use alcohol or glass cleaner on the pickup roller. It will degrade the rubber and make the problem worse.