DIAGNOSIS The float switch in the water reservoir is stuck or the sensor contacts are fouled by mineral scale.
FIX Unplug the machine. Remove the water reservoir. Flip it over. You'll see a small plastic float in a chamber. Manually move the float up and down several times. Feel for grit. Rinse the chamber with vinegar. Dry thoroughly. Reinstall the tank, plug in, and try.
IF THAT DIDN'T WORK The main control board might have a faulty sensor reading. Perform a hard reset. Unplug the machine. Hold the power button for 15 seconds. Plug back in.
HEADS UP Always use filtered or distilled water. Tap water minerals gum up that float switch fast. This is the number one cause of 'add water' errors on these units.