If a Mac has been upgraded with an OWC Aura SSD, it may fail to install to High Sierra, erroring out with: “macOS could not be installed on your computer. An error occurred while verifying firmware.”
OWC claims this is limited to certain model MacBook Airs and Mac Pros, but owners of MacBook Pros have also reported the problem. See this OWC blog post for more information. They are working with Apple towards a fix.
If you have an OWC Aura SSD and are dead set on installing High Sierra, there is a confirmed workaround. Temporarily replace the Aura SSD with the original Apple storage, boot from the Apple storage, then install High Sierra. The High Sierra install will automatically apply the needed firmware update. Afterwards, replace the original Apple storage with the Aura SSD, boot from the Aura SSD, then install High Sierra.