Diagnosis
The robot can't find its way back to the Clean Base. This is usually a navigation issue, not a battery problem. The most common culprit is something blocking the robot's top-mounted camera or the Clean Base's own infrared beacon.
How to Fix It
First, check the Clean Base. Make sure the front is completely clear for at least 3 feet on either side and directly in front. Wipe the two circular infrared windows on the front of the base with a dry microfiber cloth. Next, on the robot itself, use the same cloth to gently clean the circular camera lens on the top casing. Avoid using cleaners that can leave a film.
If That Didn't Work
The home location might have been forgotten. In the iRobot app, go to the map of your home, tap the three dots for the specific map, and select "Update my Smart Map." Run a mapping run to re-establish the home base's location.
Heads Up
This error often pops up if you move the Clean Base to a new spot. The robot's map is tied to the base's original location. If you move it, you need to update the smart map or the robot will get lost.