Hi John,
This error means that the Firesight aka Defense center isn't connected to sensor. If you navigate to device on DC , you would probably see the device(sensor) as disabled.
This might be due to communication issues. Please check if the sensor is powered on and working and there is no network issue. If no network issue , try to test the connectivity from sensor CLI to management center.
Are you receiving any health alerts as well on the management center ?
yogesh
