Diagnosis
The float switch in the internal condensate pan is stuck in the 'up' position, tricking the unit into thinking there's water when there isn't. Dust, a slimy biofilm, or a misaligned float can cause this.
How to Fix It
Unplug the AC. Locate the internal drain plug at the back-bottom of the unit and remove it to let any tiny bit of residual water out into a pan. Tilt the unit back gently. You might be able to see the small white plastic float in its chamber. Use a thin, flexible tool like a zip tie to very gently prod the float up and down a few times to free it. Reinstall the drain plug, set the unit level, and plug it in.
If That Didn't Work
The float switch mechanism could be faulty. You may need to access it from inside by removing the rear service panel to check the connection and the switch itself for damage.
Heads Up
Always unplug the unit before messing with any water drainage or internal components. And keep that area level when you run it—if it's tilted, the float won't work right.