When is this message occuring? If it is during install, this can mean that you have a bad ISO, or a bad burn. Make sure you check the MD5sum of the ISO file you downloaded. If you have the ISO on a windows computer, use either this app
or this one
to check the MD5sums against the values of the contents of the slackware-x.x-install.iso.md5 file that can be found on the mirror site you got your ISO from.