Diagnosis
The float switch in the bucket housing is either stuck in the 'up' position or the sensor that detects the bucket is misaligned. Dirt, debris, or a slightly warped bucket can cause this.
How to Fix It
Unplug the unit. Pull the bucket out completely. Look inside the empty bucket compartment—you'll see a small, usually white, plastic float on a hinge at the back. Manually move it up and down a few times. Feel for any grit or obstruction. Use a can of compressed air to blow out any dust around the float and its chamber. Reinsert the bucket firmly until you hear a solid click.
If That Didn't Work
Check the bucket itself. Look at the bottom where it interacts with the float. Make sure it's not cracked or warped. Also, ensure the bucket is sitting perfectly level on the floor; if the unit is on an uneven surface, the bucket can sit crooked and not depress the float fully.
Heads Up
Never try to bypass or tape down the float switch to run the unit continuously. That's how you end up with 70 pints of water on your floor when the internal overflow pan fills up.