Diagnosis
The scanner can't detect that the document cover (the lid) is fully closed. There's a small sensor, usually near the hinge, that gets blocked or misaligned.
How to Fix It
Check for anything preventing the lid from closing completely—a paper clip, a curled corner of a book, or even the scanning cable tucked in the wrong place. Open and close the lid firmly a few times. Power cycle the printer. Also, make sure you're placing the original face-down correctly against the alignment mark.
If That Didn't Work
The sensor might be dirty. With the printer off, look along the inner edge of the scanner glass frame, near the back hinge, for a small plastic nub or window. Gently clean it with a dry cotton swab. Don't press hard.
Heads Up
If you're using the Canon ScanGear software, try the simpler Canon My Image Garden app instead. Sometimes the driver software gets confused and throws this error even when the hardware is fine.