Why is the check light on my dehumidifier flashing?

This could mean that the dehumidifier is being used at too low a temperature (below freezing), too high a temperature (above 40°C) or that the air flow is being blocked. 

If the temperature is within the normal operating range of 1-40°C, then make sure that the filter is clean and that it is at an appropriate distance from the wall to allow the air to flow through (at least 30/40cm - refer to your dehumidifier's manual for specific instructions).