Unlocked boot loader ADB issue

I am trying to downgrade my Nexus 6P from Oreo 8.1 to 7.xx. I’ve unlocked the boot loader and can communicate with the device through Google’s adb fast boot command line which works fine (up to a point). I get this far, and it fails every time. Here are the files I am trying to flash. Is there something I am missing? I’ve installed the Google USB drivers too, as I thought that was the issue.