Diagnosis
The rubber stopper/plug in the drain port isn't fully removed, or the hose isn't seated correctly, creating an airlock. The port is on the back, near the bottom.
How to Fix It
Unplug the unit and pull out the bucket. On the back, you'll see a round rubber plug tucked into a recessed port. You need to pull this plug completely out—it's not just a twist-cap. Then, push your vinyl drain hose (not provided) firmly onto the plastic nipple. The hose must run straight down to the floor drain with no kinks or upward loops.
If That Didn't Work
If water is still backing up into the bucket, there's likely an airlock in the hose. Detach the hose, fill it with water from your sink to prime it, then quickly reattach it to the unit to create a continuous siphon. Also, check that the hose path is a steady downhill slope.
Heads Up
The bucket must still be in place for the drain to work. The internal pump drains through the bucket's outlet. If the bucket is removed, the water has nowhere to go and will overflow inside the machine.