Diagnosis
The magnetic sensor in the dock is not detecting the tank. This is almost always caused by the tank not being seated at the correct angle or a film of debris on the sensor window.
How to Fix It
Remove the clean water tank. Look at the rear of the dock slot. Locate the small rectangular sensor window. Wipe it clean with a dry microfiber cloth. Reinstall the tank by placing the bottom in first, then tilting the top back until you hear a distinct double click, not just one.
If That Didn't Work
Check the corresponding magnetic tag on the back of the clean water tank itself. Ensure it is present, clean, and not damaged. If the tag is missing, the sensor cannot detect the tank.
Heads Up
Do not use the vacuum without the clean water tank correctly installed. The roller will not receive moisture and can burn out from dry friction.