Hi Don, your model has a little misalignment in it which seems to have created a kind of micro shelf in this area here:



Zooming in there, there is a little teeny tiny edge that's only about 0.001 units long in there:



That's probably going to be what's hosing the filleting from working, although also the upper part of where you want to fillet where the surfaces become tangent to one another may be another problem area, since that's kind of a special zone where fillets degenerate down to nothing.

But I'd try slicing off the bottom by just a bit with a line to get a cleaner bottom face structure and that may help.

- Michael