DIAGNOSIS The float switch inside the bucket well is stuck in the 'up' position, tricking the unit into thinking there's still water in there when there isn't.
FIX Unplug the unit first. Pull out the bucket. Look inside the empty bucket cavity—you'll see a small, usually white, plastic float on a hinge. Gently press it down a few times to make sure it moves freely. Then, use a cotton swab dipped in distilled white vinegar to wipe the sides of the well and the float itself to remove any mineral scale or gunk that's jamming it. Let it dry, plug it back in.
IF THAT DIDN'T WORK The float switch mechanism might be physically broken or the sensor wire connecting it to the main board could be loose. You'd need to open the lower rear panel to check the connection, which is a bit more involved.
HEADS UP This often happens if you let the bucket overfill or if you have very hard water. Using the continuous drain hose is the best way to avoid this issue entirely.