Hi alexito, also be aware that on some places of your model the edges are split into 2 edges, not just one edge, like here:



So in those spots make sure you are picking both the bottom and top edges to fillet, if you only pick just one half then the fillet will not work.

Then as others have mentioned there isn't enough space there to get too much larger than 0.5 or so.

Those edges got split into half during the boolean due to the placement of the "seam" edge of your sphere. Your sphere looks like this:



Notice how the seam of the sphere is running along the side area? That is a result from drawing the sphere in the Front view initially - if you draw it in the Top view the seam will be oriented vertically rather than horizontally and it will be a little bit more out of your way as far as interacting with the other pieces you are booleaning...

It can be a little more convenient to orient a sphere or cylinder so that its seam is somewhat more out of the way from the other objects you are going to combine with it.

- Michael