Diagnosis
Yeah, this one's common. It's usually a false positive from a tiny scrap of paper you can't see, or more often, a stubborn piece of torn label or receipt paper that shredded and is wrapped around a roller deep inside.
How to Fix It
Unplug the printer. Open the rear access cover. Use a flashlight and look way back in the paper path, especially around the rubber rollers. Feel for any debris. If you see a scrap, use tweezers to pull it straight out—don't yank. Close the cover. Now, open the scanner unit (the whole top), and check the area under it, near the printhead carriage. A tiny scrap can hide there too.
If That Didn't Work
The sensor itself might be stuck. With the printer unplugged, locate the little plastic paper detection flag (usually a small white or orange lever) along the paper path and gently wiggle it with a pen to make sure it's moving freely and not stuck in the 'jammed' position.
Heads Up
Never use excessive force or metal tools deep inside. You can easily damage the delicate printhead or scratch a roller, which will cause worse problems.