Diagnosis
This is almost always a misaligned or stuck float switch in the bucket compartment. The little plastic tab that senses when the bucket is full isn't dropping back down.
How to Fix It
First, power the unit off and unplug it. Pull the bucket all the way out. Look inside the empty compartment on the left side—you'll see a small, usually white, plastic float. Gently press it up and down with your finger a few times. You should feel it click smoothly. Sometimes it just gets hung up. Then, reinsert the bucket carefully, making sure it slides all the way in without hitting that float. Plug it back in and power on.
If That Didn't Work
Check the bucket itself. Make sure it's seated perfectly flat on the floor and that the little notch on the front is clicking into the unit's base. If it's even slightly crooked, it can hold the internal float up.
Heads Up
Never try to run it with the bucket removed to bypass this. You'll flood your floor. The safety switch is there for a reason.