DIAGNOSIS The float sensor inside the side tank is stuck, usually from mineral scale buildup or a piece of debris.
FIX Remove the side tank. Look at the bottom—you'll see the float mechanism in a little well. Gently move the float up and down with your finger to see if it's stuck. If it is, fill the tank with a mix of warm water and white vinegar (about 4:1 ratio) and let it soak for 30 minutes. Then, vigorously slosh the water around in the tank to try and dislodge any scale. Rinse thoroughly with clean water.
IF THAT DIDN'T WORK The issue could be with the magnetic sensor on the ice maker base where the tank sits. Wipe the round sensor area on the base and the corresponding spot on the bottom of the tank with a dry cloth. Any water droplets or debris there can mess with the signal.
HEADS UP This is almost always a scale issue. If you have hard water, you'll need to clean that tank sensor more often than the recommended schedule. Using filtered or softened water in the side tank makes a huge difference.