DIAGNOSIS The dirty water tank is either full, not seated correctly, or the float sensor inside it is stuck, telling the station the tank is full when it's not.
FIX 1) Pull out the dirty water tank and empty it. 2) Before putting it back, rinse it out thoroughly. Pay special attention to the small rectangular float chamber on the side—shake water around in there to make sure the little plastic float moves up and down freely. 3) Dry the tank's electrical contacts (the metal pins on the back) and the contacts inside the station with a dry cloth. Re-seat the tank firmly until you hear a click.
IF THAT DIDN'T WORK There might be a physical obstruction in the drain hole in the cleaning tray where the robot parks. Unplug the station, pull out the tray, and check for debris blocking the hole that leads to the dirty water tank.
HEADS UP If you let dirty water sit in the tank for too long, mold or sludge can jam that float mechanism permanently. Get in the habit of emptying it after every couple of mopping cycles.