an internal error occurred while preflighting your volume for APFS conversion

Is this on an SSD, a hard drive, or a Fusion Drive installation? If's it's a hard drive or Fusion Drive then the on-disk format used for that in the High Sierra beta was dropped before the final release and is unsupported. The golden master of High Sierra only supported APFS on SSDs.

So if you're trying to upgrade a High Sierra beta install to Mojave it's not surprising that it's confused. If you somehow got a release build of High Sierra to run on an APFS beta HDD or Fusion setup, I'm amazed it's worked for the past year. (APFS on Fusion repeatedly munged the filesystem badly on my iMac last summer, so I can't say I was too shocked when they pulled it after a few betas.)