Is the cube you want to cut through selected? It doesn't know where to cut when you don't select anything as far as I know. Could be wrong tho, I haven't used it in a few weeks.
I highly suggest you to post this in the Boxcutter discord or blenderartist thread, which is regularly checked by the devs. I love muh Polycount and would like to see more Blender threads here, but when it's about an addon that has an enormours help service elsewhere...