I'm trying to create a solid out of this surface which works ok when I use extrude. However I then want to fillet its side which I can't seem to do. Nothing happens when I apply fillet.

turning on show points seem to show that maybe the program has got mixed up as the cage is huge. I've tried joining the model but its has no effect.

Could someone tell me how to freeze the model? Or why I can't fillet it?

Any help much appreciated.