DIAGNOSIS The robot thinks its main brush is jammed. This is almost always because hair or a thick thread has wound itself super tight around the brush ends, blocking the rotation sensor.
FIX Flip the robot over. Press the two tabs on the main brush cover to remove it. Pull out the main brush. You'll need to cut or unwind whatever's wound around the plastic ends near the bearings. Clear the brush housing of any debris too. Reinstall the brush, making sure it clicks into the gear on both sides, and snap the cover back on.
IF THAT DIDN'T WORK The error can also be triggered by a faulty brush motor or its sensor. After clearing the brush, try a hard reboot: hold the power button on the robot for about 10 seconds until it fully shuts off and restarts.
HEADS UP This error pops up a lot if you have long hair or pets. Make a habit of checking and cleaning the main brush ends every couple of weeks, not just the brush itself.