This could be a bug in the program, but to root it out and maybe offer a workaround, we’ll need more details. Does the error happen as soon as you try the first file, or does it happen after a number of files? What file type(s) does it happen with? A list of steps from when you first open the CON Creator to when you see the error will be helpful.

If you’re trying to add files to the left drag-and-drop window, that window is only for folders, and so adding files there will not work.

If you’re selecting the “Root” directory and then adding files to the right drag-and-drop window, the “Root” directory shouldn’t have anything added to it, as everything in a properly formatted CON file is in the “songs” directory and below. I did discover a bug where if you do try to add a file to the “Root” directory, the CON Creator freezes and has to be restarted. I’ll add that to the bug list. However, I still did not see any error message, so I’m not sure I’m running into the same problem you saw.