You need to modify either the edge width or padding values as per the description below.

The edge width measurement defines the width of material to be used for the middle layers in which to place the mount holes. The center of the mount holes will be placed in the center of the edge width. This allows you to create cases with added space inside the case for other components or other creative uses. There needs to be at least two opposing sides with padding equal to or greater than the edge width in order for holes to be placed. If all sides of the case have padding greater than or equal to the edge width then the mount holes will be evenly spaced around the entire case.

If you are getting this message it means that you have defined mount holes, but you do not have two opposing sides of your case with padding greater than or equal to the edge width.

You must specify padding on at least two opposing sides equal to or greater than edge width.