DIAGNOSIS The paper size sensors in the tray or the paper guides aren't being triggered correctly, so the printer thinks you've got a different size loaded. Often it's because the little blue paper guide in Tray 1 isn't snapped firmly into the detent for the correct size.
FIX Pull Tray 1 all the way out. Empty it completely. You'll see the blue paper length guide on the right. Slide it all the way out, then push it firmly back into the slot until you feel it click into the notch marked for Letter (or A4). Do the same for the width guide at the back. Reload the paper, fan it to prevent sticking, and make sure the stack is under the metal paper clip. On the control panel, go to Setup > Paper Handling > Paper Size Detection and make sure it's set to On.
IF THAT DIDN'T WORK There's a small, spring-loaded sensor flag inside the paper path that can get jammed with paper dust. With the printer off, open the rear door and the duplexer (the flip-down unit on the back). Look for a small, usually orange, plastic lever near where paper feeds in. Gently move it with your finger to ensure it's not stuck. Blow out any dust with compressed air.
HEADS UP If you're using off-brand or recycled paper that's slightly out of spec, the printer will complain. Try a fresh ream of known brand name paper to rule that out.