Hi Dan, thanks for posting the file.

Is this the edge you're trying to fillet?


It's pretty subtle but I happened to notice these edges were looking a little darker than the others:


That's an indication that there are actually multiple edges being displayed on top of each other instead of just a single edge like you'd want to have. I zoomed in to the corner area and the problem is that there are few really skinny surface fragments in that area:


So there are actually several little tiny edges and surfaces in that back and corner area and that's what is going to interfere with getting a fillet running through there.

Maybe some of the pieces were just barely not aligned when doing a previous boolean step. It can be good to make things that you are going to boolean union to push a little ways through each other before doing the boolean instead of making things that just barely skim right over the same surface area.

- Michael