Thats not a vent style drain thats to help the Evaporator fan put air to the front,remove all your food from the freezer ! Then with a philps screwdriver remove the four screws that hold down the lower plastic freezer floor panel!
Reach in and lift up on the fan cover(be carefull not to break the styrofoam down draft tube)then take the back plate off the freezer(two screws) lift this out1
You will see the freezer coils and at the bottom you will see an alluminum-paper type drip pan,in the center you will see a drain hole!
Lift the drip pan out and look for Ice,small peas,or anything that might have gotten into that hole!!!
That hole is a drain tube that runs (inside the frame) of your refrig and down the lenght of it right down next to your compresor(into a plastic drip pan that lets the water evaporate)!
I have had sometimes to cut this tube at the jay bend to run bailing wire or other things up the drain tube to remove coffee grinds ,ect!!
After you get the drip pan out try a hair dryer or hot water first(before attempting surgery)!!
Bob