Diagnosis
The micro-switch on the chassis that detects the tank is not being engaged, or the tank's contact tab is broken. The error is triggered by the physical switch, not a sensor.
How to Fix It
Remove the tank. Look inside the robot's tank bay for a small, black plastic button (the switch). Press it with your finger. You should hear a faint click. If it sticks, spray a small amount of electronic contact cleaner around it and press it repeatedly. Reinstall the tank, ensuring it clicks firmly into place.
If That Didn't Work
Inspect the plastic tab on the rear of the water tank that presses the switch. If it is cracked or worn down, it will not press the switch. A small piece of tape or plastic shim added to the tab can restore contact.
Heads Up
Do not bypass this switch by holding it down manually during operation. The robot uses tank presence to regulate water pump operation.