DIAGNOSIS The magnetic float in the reservoir is stuck, or the water sensor contacts on the machine base are dirty.
FIX Remove the water reservoir. Check the small magnetic float on the bottom. Ensure it moves freely up and down. Clean it with vinegar if it's scaled. On the machine base, wipe the two circular metal sensor contacts with a dry cloth to remove any water spots or mineral deposits.
IF THAT DIDN'T WORK The Hall Effect sensor inside the machine base is faulty. This requires a factory reset: unplug for 1 minute. If the error persists, the main control board is defective.
HEADS UP This sensor only detects the presence of the reservoir, not the precise water level. The 'Add Water' alert triggers if it cannot detect the reservoir's magnet, regardless of actual water volume.