If you’ve been using the macOS High Sierra Beta on a Mac with a Fusion Drive you’ll want to pay attention to Apple’s announcement and instructions:
Beta versions of macOS High Sierra made a change in the disk format of systems by converting them to use the new Apple File System. The initial release of macOS High Sierra will provide support for the new Apple File System as the default boot filesystem on Mac systems with all-Flash built-in storage. If you installed a beta version of macOS High Sierra, the Fusion Drive in your Mac may have been converted to Apple File System. Because this configuration is not supported in the initial release of macOS High Sierra, we recommend that you follow the steps below to revert back to the previous disk format.
In other words, you’ll have to back up your Mac, format your drive, reinstall the final version of High Sierra and restore your data.