DIAGNOSIS Error 21 is a scanner motor or movement failure. Something is physically preventing the scanner carriage from moving, or the motor that drives it has failed.
FIX Unplug the printer. Open the scanner lid. Look for any obvious obstructions—a paperclip, piece of paper, or debris—on the glass or in the tracks. Gently try to move the scanner carriage (the unit with the light bar under the glass) from side to side. It should move freely. If it's stuck, carefully clear whatever is jamming it. Plug the printer back in and see if it initializes without the error.
IF THAT DIDN'T WORK The error can be triggered by a faulty flatbed cable or a failing motor. This requires more disassembly than is advisable for most people. As a last-ditch software fix, you can try resetting the printer to factory defaults (this will erase all network settings). From the control panel, go to Setup > Service > Restore Factory Defaults.
HEADS UP If the carriage moves freely and the error persists, it's likely a hardware failure. For Error 21, HP's typical service procedure is to replace the scanner assembly, which is a professional repair job.