Error message #43 means: "Extent of box not within map limits". You get this error message if you try to contour the map in a volume which is not covered by the map in the file. For example, if the map covers the A molecule, and you are trying to display contours in the B molecule.

You can avoid this error by calculating the map in the place(s) where you are going to want to contour them, or, conversely, by not trying to contour the map in places where you haven't calculated it, or, facetiously, by changing the error message in your O database.