Me and script don't always see eye to eye. I think it's a language barrier, or something. I'd be happy to share the file, although I'm not sure how I'd go about doing that. The Attach Image/file won't allow 3ds Max files.
"Constraint Not Completed" is the generic fail message for AssignConstraintSelection() in controllerfunctions.ms. (It's in the 3ds Max install folder.) If you are comfortable with script you can modify the function to output more debug information. If not maybe you can share your file so we can dig into it.