DIAGNOSIS The float sensor or the optical water level sensor is stuck or blocked, so the machine can't detect the water. This is a common issue with mineral deposits or a misaligned float.
FIX Unplug the machine. Open the lid and locate the small white float (usually on the side or back wall of the reservoir). Manually move it up and down its track several times to ensure it's not stuck. Then, use a cup to remove all water. Check for any small debris (like a stray ice chip) blocking the sensor or float path. Refill with fresh water to the MAX line and plug back in.
IF THAT DIDN'T WORK The sensor contacts might have scale. Perform a full descaling cycle with vinegar or cleaner as outlined in the cleaning question. The acid can dissolve mineral buildup interfering with the sensor.
HEADS UP Always use the machine on a level surface. If it's tilted, the float sensor won't read the water level accurately, causing this false alarm.