You have six control points. one each corner in White and the two Blue ones.
Try hovering over each in turn then click-drag.
The corner ones are for resizing; the blues for curvature.
I have just tried as small as 5%.
If you can do the Whites, you can do the Blues.
If not, you need to close the app and re-open as Administrator and retry.
Acorn
Bookmarks