I am also having a problem with this tool. I am working on a MacBook, MacOS 10.15, then the 11 Beta. The fault appears an intermittent one. With some images, the tool works as expected. Other images - the tool does nothing apart from produce a movable grid. That is, it has no effect on the image. This can happen between photographs taken during the same session on the same (iPhone) camera. Quitting the program and going back in sometimes works. The kind of source file doesn't seem to make a difference, but performance is a little patchy.
