Roborock robot vacuum (mopping models) reports “Error 21” / “Water tank error” or “Water tank not installed correctly.”

Safety first: Power off the robot and remove the water tank. Do not spill water into the robot body.

Step-by-step:

• Inspect tank and contacts: Check the electrical contacts or magnets on the tank and robot body. Wipe them dry and clean with a soft cloth.
• Check latches: Ensure the tank’s mechanical latches are intact and not cracked.
• Reinstall tank: Slide or click the tank back into place firmly until you hear or feel it lock.
• Test mopping mode: Power on the robot and start a mopping task to see if the error clears.

If the robot still does not detect the tank: The tank’s internal electronics or the robot’s detection circuit may be faulty. Replacement of the tank or internal board may be required.