Mohave does not support booting from RAID volumes
With the release of 10.14 Mohave, users can no longer startup from RAID volumes. This includes any SoftRAID volume and Apple RAID volumes.
There is no workaround for this, and we do not expect there to be a solution going forward.
We had several discussions with Apple to see if they would re-enable booting in the future, but the chances are slim to zero.
Prepare to migrate your startup volumes to Apple standard volumes before attempting to install Mohave.