Hi sosodef, what fillet radius are you trying to use? It looks like probably only a pretty small radius will fit in the space you've got there. Try going down in increments of 1/10 like try a radius of 0.1 or 0.01 and see if that produces a fillet. If it does then it means your original radius value was just too big to fit in some of the corner areas of your shape.

If you need more help please post the 3DM model file that you're trying to fillet, that will make it possible to take a look at your geometry and maybe give some better tips.

- Michael