What says "unable to create directory"? If it is World Editor then there must be some problem with creating the temporary files for the model during the save process. These temporary files are used as sources when building the map MPQ archive.
Possible causes could include third party applications blocking the file, such as antivirus or cloud backup software, the drive having insufficient disc space to house the temporary file, the temporary file containing illegal characters not supported by the file system or even that the resulting file system path length is longer than the maximum supported file system path length (260 characters for normal Windows applications).