error is given because the folder is both on the source/left and
destination/right but only the contents of one of them could be scanned.
Because of this, the files in those folders cannot be compared. Thus, the files will be skipped, because if their properties (or presence versus absence) cannot be compared, it cannot be determined which rules/priority should apply to which file.
file will also contain an error message on why that folder (or one of
its parent folders) could not be scanned, e.g. Access Denied.
Bear in mind that if a folder's contents cannot even be scanned, it is virtually certain that files cannot be read from or written to such a folder anyway.