Diagnosis
The scanner carriage is jammed. This usually happens if something is blocking its path under the glass or if the scanner lock wasn't fully released during setup.
How to Fix It
Unplug the printer. Look for a small white plastic transport lock or tape on the left side of the scanner assembly, under the lid. It should have been removed, but sometimes a piece breaks off. Remove any remnants. Then, gently try to manually slide the scanner unit (the part with the light inside) from side to side. Don't force it. If it moves, center it.
If That Didn't Work
There might be an obstruction like a paperclip or a dropped screw on the scanner bed. Shine a light under the glass while manually moving the carriage to spot anything. You may need to partially disassemble the scanner cover to remove it, which is a more involved fix.
Heads Up
If you hear a grinding plastic sound when you try to move it, stop. The drive belt may have jumped its track or a gear is stripped. Forcing it will cause permanent damage requiring a service technician.