On the pixel 6, make sure you're in Fastboot
Mode (bootloader), and on the p4 you don't have to change any usb setting (the flasher will see it as long as you are using Chrome on PC or Vanadium on Pixel 4a). It will say "charging connected device" - that's ok. Just move to the chrome browser and begin the flash process.
Whenever you get stuck on the pixel 6, just hold volume down and power (no matter what you see on the screen), until the phone boots into Fastboot Mode. You can get to this screen no matter what is happening on your phone (as long as it's charged and not plugged in).
So now that you're in Fastboot, connect it to either your PC or Pixel and flash it again. Remember that the chrome on PC or Vanadium on Pixel 4a is required (and ensure you're not in incognito/private mode).
If you still get the corrupt message, go into recovery mode on the p6 and do a factory reset (you'll see 'no command' - that's ok, just hold power and quickly press volume up). Then pick wipe/reset and flash again.
If you're still stuck, go back to stock (flash.android.com) then re-flash Graphene.
Just remember that during the flash process, the phone will restart into fastbootd - make sure to not touch the phone during this time until the flash process is complete.