I found using double partition and boot into Yosemite and installed Parallels 13 under Yosemite; the installer immediately found the BootCamp partition with Windows 10 and installed with no problem. I found this works faster and uses only app 260megs hard disc space for the VM (already have allocated HD space on the WIN 10 install). I found that installing under High Sierra (with difficulty) uses over 14 gigs of HD space and is not VM capable. Running under Yosemite is allowed since your copy of WIN10 is already activated. I'll do this until there is a install fix with High Sierra.