DIAGNOSIS The most common cause is air trapped in the internal water tank's tubing, blocking the flow. It happens often with a brand new unit or if you've let the tank run completely dry.
FIX You need to prime the water pump. 1) Remove the water tank from the robot. 2) Fill it completely to the max line, seal it, and then turn it UPSIDE DOWN. 3) Gently squeeze the tank a few times. You should see water droplets come out of the mopping module's nozzle. 4) Keep it upside down, reattach it to the robot, and start a mopping-only cycle. The vibration should push the rest of the air out and water will start flowing.
IF THAT DIDN'T WORK Check the mop pad itself. The grey, rubberized 'water guide' plate must be installed correctly between the cloth and the robot. If it's missing or upside down, water can't wick through to the cloth.
HEADS UP Only use water in the tank. Cleaning solutions or vinegar can clog the tiny nozzles and valves in the mopping module, which is a much harder fix than clearing an airlock.