DIAGNOSIS The dock's internal water pump is likely clogged with sediment or scale. No water flows to the robot's mop washing area.
FIX Use only distilled or softened water. To clear it, empty both tanks. Remove the clean water tank. Look inside the dock's water intake port for debris. Use a small syringe with distilled water to backflush the port. Run a manual mop wash cycle with a vinegar/distilled water mix (10% vinegar) to descale.
IF THAT DIDN'T WORK The pump motor or its filter screen inside the dock is severely blocked. This requires partial disassembly of the docking station.
HEADS UP Hard water will cause this repeatedly. Never use chemical descalers not approved for plastics, as they can damage internal seals.