Hi stinkie, thank you very much for your order!

The switch has a pretty narrow area on it here:



There really isn't very much space in that area to fit fillets - here I have done a fillet of radius 0.03:



Zooming on that narrow region, you can see that there isn't room for much more of a larger radius than that:



Much larger than that and that side piece will be totally eaten away - that will cause the fillet to fail to calculate.

It is often times surprising how much space a fillet will take up, if you are not getting a result try something really pretty small like 0.01 or 0.005 and see if that works, then gradually increase the size.

Hope this helps!

- Michael