Hi simba - the fillet will get confused with the surfaces touching each other on the leading edge side. So to build the fillet you will need to cut away a large chunk of the wing and also probably use ShrinkTrimmedSrf on them as well so you have surfaces configured as in the attached version. You do that just temporarily to form the fillet, then bring the fillet back into the full original shape.

Note that when you do the fillet it will make a weird trim on the surfaces since the fillet does not quite run the entire length. But that's ok because you just want the fillet surface itself so just do Ctrl+C on the fillet surface, then go back to your original model and Ctrl+V to paste it in there then use the edges of the fillet for trimming the original airfoil.

- Michael