RoundCorner question

Can anyone tell me how to fillet this face/ edge using RoundCorner extension?
I keep getting the circle w line symbol (in spite of being inside of the group and even exploding)
This is a taper/ extruded semicircle (made by using Curviloft) fyi
Thanks!

It would help if you could upload a copy of the SketchUp .SKP file to the forum, so that other users could see exactly what is going on. (Drag the .SKP file into a new reply, or use the upload icon, which has an up-pointing arrow.)

Do you want to apply a fillet to the semi-circular end face of the object? I would double-click on the face to select the face and its edges, then shift-single-click on the face to de-select the face. Then invoke the RoundCorner extension - hopefully it will display a free check-mark when you point the mouse in free space, signifying that the selected edges can be modified.