5100 — something is blocking the print head
This one sounds worse than it usually is. Canon's own instruction is to cancel the print job, switch the printer off, remove whatever is stopping the print head holder from moving, and switch it back on. On a printer you've just unboxed, the blockage is almost always packing tape or an orange plastic clip nobody removed. On an older machine, look for a shred of paper under the carriage rail, and check the thin plastic strip running behind the carriage isn't smeared with ink. Canon warns against touching the components inside, so work gently and stay in the paper path.
Related codes: 5100