Ok this means the temperature is dropping inside the freezer. The light comes on around -13 degs so its still cold enough to freeze but will soon get worse im afraid.
What it is on this mostly is a blocked drain hole causing too much ice to build up and it effects the air flow around the freezer and gives this fault.
You have 2 choices here to fix this. Either empty the top half of the freezer.
Then look at the top of the freezer at the front and you will see 2 screws holding the cover to the roof either side.
Remove them screws and then pull the cover down and off. You will need to disconnect the wires first for the heater in the tray.
Once that's out clear all the ice in there. Also clear the ice from the drain hole at the back which you will see. Now this way sounds easyer then it actually is. As if there is ice then this makes it harder to remove the tray but it will come out.
Or the other way is a full defrost for 24 hours. Empty the freezer and turn it off and leave the door open for 24 hours and this will allow it to defrost itself from there.
Then turn it back on and reset the red light if lit again and then retry it and im sure that will sort it from there for you.
I see this fault a lot on these and its nearly always a blocked drain hole that causes this fault here.