Hi mark, yes normally the edges to be filled would be touching each other at sharp corners, although it may be able to work if they are not.

But also it expects there to be at least 3 edges, it definitely won't work if there's just one single edge like it looks like you may have there. I'd be able to tell about that more easily if you would post the 3DM model file that has your object in it instead of just a screenshot.

You can try to cut that edge up into multiple separate edges using Edit > Trim with the "Add trim points" option inside Trim, then that may work but probably that kind of case with smooth sides isn't going to give very good results.

- Michael