DIAGNOSIS The mop plate microswitch isn't engaging. The spring-loaded tab on the plate or the sensor lever inside the dock is stuck or worn.
FIX Power off the robot. Remove the mop plate. Check the small plastic tab on the plate's top edge. Clean any debris. Manually press the small black lever inside the robot's mop plate slot a few times to free it. Reinstall the plate with a firm click.
IF THAT DIDN'T WORK The microswitch itself may have failed. This requires a mainboard replacement. Contact Roborock support.
HEADS UP Forcing the plate can break the lever. Never run the robot with this error, as it may try to retract a non-existent mop and damage the mechanism.