Hi Shazbot, which area are you trying to fillet and what fillet radius are you using?

Your shape is very narrow, this area here is only 0.03 units across:



Because that area is so thin, it will limit the size of fillet that can be placed there to a small value, like around 0.01 units or so. Are you maybe trying to place fillets larger than that?

One thing you should try in general when you run into fillet failures is to keep trying smaller values like go down to 1/10 or even 1/100 of the size that you were previously trying just to see what that does, it's fairly easy to misjudge how much space a particular fillet will actually eat away in the model.

On your model when I load it and don't do any alterations to it, I can make a selection like this:



And then doing a fillet of radius 0.01 gives me this result here:




You won't be able to go much larger than that, because notice how the way the fillet works is that the fillets in these areas here eat away part of that thin zone:



You will be limited to only place a fillet that's not so large that it totally eats all that area away and things from opposite sides start running into one another.

If you want to build things with a large fillet radius you would need to make the objects made up of somewhat thicker pieces so that there is actually enough room for a larger radius fillet to fit in that area of the model.

Please let me know if you were having problems with something else other than that.

- Michael