You must have patched boot img with dm-verity and forced encryption disabled to flash anything or modify /system. Use rootkernel method posted in guides forum.
Note that I don't know anything about this stuff, I just followed a guide and it happened to work. I couldn't tell you anything technical.
Update - now working on O with root - https://forum.xda-developers.com/x-c...overy-t3747479
Note - decrypt data not working, but system mods working normally.