Hi Tom - selection does not persist through file saving and reloading so I'm not sure which edges you are trying to fillet (EDIT: oops I see it's the dark yellow style that you've set up, I will take a look).

But a couple of things to do - right now you have 4 separate objects and you probably need to combine them into one single object using Construct > Boolean > Union.

Also the filleter is probably not going to like the outer surface being split into different pieces rather than one single surface, I'd probably try to rebuild that outside surface as just one large surface instead of in separate pieces like you currently have.

I'll try to set that up for you.

- Michael