This means the electronics are not sensing rotational feedback from the fan when it's supposed to be turning.
This usually means the fan is frozen up, which can be caused by the door being left open or if there is a gap in the door gasket when it's supposed to be closed. It could also be fan motor failure.
If you wish to defrost the unit, you can, otherwise contact us to request service.
In addition to defrosting, make sure the unit is not packed so full that it prevents the doors from closing completely. Check that the door gaskets make a full contact connection all the way around the door.