I have a Frigidaire side by side model # FRS26R2AW8 that does not freeze. It cools to about 36 to 40 degrees Freezer compartment.

I removed the back cover (inside the freezer compartment) and noticed the evaporator coil frosts up very quickly on the top 1 or 2 coils and begins to build up ice quicly on those coils but does not appear to get cold lower on the coil.

There is a black "tube" connected to wires that runs through the middle of the evaporator coil that I think is the heater. I did not notice it being hot when I removed the cover to inspect the coil.

I'm assuming there is a leak in the sealed system. Can it be anything else that causes the freezer compartment to not freeze?

The defrost heater only operates during the defrost cycle. What you have described sounds like low refrigerant. What type of refrigerant does it use?