Diagnosis
The float sensor in the internal condensate tank is stuck in the "up" (full) position, or the contacts are dirty.
How to Fix It
Power off and unplug the unit. Tip it slightly back and then gently from side to side. You're trying to dislodge the small float inside the tank. Plug it back in. The light should go off if the float dropped.
If That Didn't Work
Mineral deposits from evaporated water can jam the float. You'll need to access the internal tank to clean it, which involves removing the rear lower panel. If you're not comfortable with that, contact support.
Heads Up
Don't ignore this and just run the unit. If the sensor is faulty and the real tank *does* fill, it could overflow inside the unit and cause water damage.