One of the new features in Apple’s macOS High Sierra is their new APFS file system (Apple File System), this is only supported by Solid State Drives on the Mac at the moment.
Apple’s own Fusion Drives are not supported by the APFS at the moment, although Apple has now confirmed that this is something they plan to add in the future.
The APFS was supported by Fusion Drives during the release of the macOS High Sierra betas, although this support was removed before the final release. Apple’s Craig Federighi has now confirmed that they company will bring this to Fusion Drives in the future.
As yet there are no details on when this support will be added, Apple obviously had an issue with the APFS and Fusion drives, as this explains why it was available the betas and not the final release.