High Sierra won't install: for the boot disk it says: "This disk cannot be used to start your computer" I have a macbook pro late 2012, single stock SSD, Sierra installed with latest updates.  I am trying to install High Sierra.   

When I go to select the disk to install on (the current, normal boot volume), the installer refuses, with the error: "This disk cannot be used to start your computer"

Disk Utility First Aid can find nothing wrong.  The computer boots normally just fine with Sierra.

Volume name : Macintosh HD
Volume type : Logical Volume
BSD device node : disk1
Mount point : /
File system : Mac OS Extended (Journaled, Encrypted)
Connection : PCI
Device tree path : IODeviceTree:/PCI0@0/PEG2@1,2/SSD0@0/PRT0@0/PMP@0
Writable : Yes
Is case-sensitive : No
File system UUID : 308F1BD9-CF92-335C-B437-D47C941E7D76
CoreStorage UUID : 0E3E3C70-5FE7-484E-826B-828CDA8F3471
Parent CoreStorage LVG UUID : 13CB696B-1760-4072-A883-0ED19EFBEBF1
Volume capacity : 374,756,999,168
Available space (Purgeable + Free) : 168,884,392,284
Purgeable space : 9,433,788,764
Free space : 159,450,603,520
Used space : 205,872,606,884
File count : 1,735,041
Owners enabled : Yes
Is encrypted : Yes
System Integrity Protection supported : Yes
Can be verified : Yes
Can be repaired : No
Bootable : Yes
Journaled : Yes
Disk number : 1
Media name : Macintosh HD
Media type : Generic
Ejectable : No
Solid state : Yes
S.M.A.R.T. status : Not Supported
Parent disks : disk0s1
You must turn off filevault before install HS...