Hi eric, so is this the edge that you are trying to fillet:



I'm not exactly sure what the problem is there, but it may be some kind of really small tangent deviation right at the close point of that closed loop.

Are you possibly making these circular surfaces using the Sweep command? For things that are perfectly circular try using Revolve instead, it will generate more precise circular geometry.

Anyway, you can use the surface/surface filleting method to build the fillet for this case even though the edge-based filleting fails.

To use surface/surface filleting, you need to break the pieces into 2 individual surfaces and then run fillet.

So to do that select these 2 faces:



Then run Edit > Separate once, which will break those 2 faces off of the main solid (but they are still connected to each other), and then run Edit > Separate a second time to break that piece apart into 2 totally individual surfaces.

Then you can run Fillet on those 2 surfaces and it is able to build a fillet there for you:



Then select all those pieces and use Edit > Join to glue everything back into a solid.

If you're stuck with following this part, just let me know what radius you would like to have there and I will make it for you.

- Michael