Diagnosis
The robot thinks the mop pad assembly is missing, even when it's attached. This is almost always a communication problem with the small metal contact pins on the robot's underside that need to touch the pad's connector.
How to Fix It
First, power off the robot. Remove the cleaning pad and its plastic mount. Take a cotton swab with a tiny bit of isopropyl alcohol and gently clean the three small, circular metal contact pins on the underside of the robot. Let it dry completely for a minute. Then, reattach the pad mount, ensuring it clicks firmly into place. Power the robot back on.
If That Didn't Work
The plastic mount itself might be the issue. Inspect the small metal contact plate on the back of the mount for any corrosion or debris. If it looks damaged, you'll need a replacement mop pad mount (iRobot sells them).
Heads Up
Never run the robot with a wet mop pad already installed when you start a job. The docking station's auto-fill system needs to detect a dry pad first, or you'll get errors and potential leaks.