In case you have trouble unlocking your Pixel 6a (greyed out "OEM unlocking" button in the developer options), this is what worked for me:

  1. Start the stock Google Android
  2. Connect the phone to the internet (no need to log into any Google accounts etc.)
  3. Update the OS to the latest version (at least the build from June)
  4. Make a factory reset
  5. Activate the developer options
  6. Connect the phone to the internet again (this seems to be important)
  7. The unlock button in the developer options should work now

Good luck!

I can verify this worked on my Pixel 6a.

A note — if you have the unlocked phone and intend to use a Verizon SIM card, do not insert the SIM card prior to enabling OEM Unlocking.

NOTE to any Pixel 6th gen users looking to unlock their bootloaders DO NOT update to Android 13 if you wish to flash GrapheneOS. Android13 brings with it anti-roll back features to the bootloader preventing being able to roll back from 13 to a 12 or 12.1 version and any attempt to do so will brick your device.

ONLY FLASH the latest 12.1 release prior to flashing GrapheneOS. Otherwise Pixel 6th Gen users who already have Android13 will have to wait for the completion of the port from 12.1 to 13 to be completed sometime between the end of August through September minimum.