DIAGNOSIS The float sensor in the water tank is either stuck, dirty, or the electrical contacts on the tank/base aren't making a connection. This is a frequent sensor glitch.
FIX Remove the water tank completely. Check the bottom of the tank—there's a white float inside a clear column. Gently tap the tank to see if the float moves freely up and down. Rinse the tank and the float area thoroughly. Also, wipe the three metal contact pins on the machine's base with a dry cloth. Reinsert the tank firmly until you hear a click.
IF THAT DIDN'T WORK The sensor itself might be faulty. Try a hard reset: turn the machine off at the side switch, unplug it for 2 minutes, then plug it back in and power on. If the error persists, the issue is likely a failed sensor requiring service.
HEADS UP Don't use vinegar or harsh cleaners in the water tank, as they can damage the float sensor. Use only recommended descaling solutions through the machine's internal system.