Homing error Z #26301 (MK4S) #13301 (MK4) #21301 (MK3.9)

A fault in the Loadcell can trigger a homing error. Ensure that the printer is fully cooled down, go to LCD Menu - Calibration - Diagnostics - Loadcell Test. When tapping the nozzle, use a hard object for it. This is only to check if the Loadcell is responsive or not.

Make sure that the extruder is not exerting force to pull down the filament. If you are not sure, try to unwind it a small amount on the spool when testing the Z-axis.

In case you have an MMU, Enclosure, or custom setup where the filament is fed to the extruder by a PTFE tube, make sure that the extruder does not have to exert force to pull down the tube.

Check if the trapezoid nut has the correct orientation.

If no issues are found, open the side fan and make sure that the thumb screws that hold the heatbreak are not over-tightened. Always tighten these screws by hand, never with a tool.

Also, try loosening the screws shown in the picture below, and re-tighten them, making sure that they are not over-tightened. After that, go to LCD menu -> Control -> Run full selftest.