While loading to the extruder, as soon as the filament sensor detects a filament the printer performs an automatic loading check. If the check fails, this error is shown. Each failed load might be accompanied by the printer beeping.
The printer’s automatic loading check loads approximately 40mm of filament and then unloads 30mm of it. If the filament sensor stops detecting a filament at any time during the process, the error is shown. This process is very sensitive, so making a precise Filament sensor calibration is needed in order to prevent print defects.
While your MK3S+ is performing the loading check, solid blocks should be generated on the bottom of the LCD representing the IR filament sensor detecting a filament during the process.
If there is a line instead of any blocks (drop in the sensor’s reading), this means that the sensor stopped detecting filament at that moment, resulting in a failed loading check. The printer then retries the load. If the cut function is enabled, the MMU tries to cut a filament tip. If the drop while loading persists even during the retries, the printer unloads the filament and shows the error.