I did it yesterday. Just for testing. Attention!!!
FPOS installation will fail, because of the avb_custom_key.
You will get more trouble then now, if you start installing FPOS again. My mention.
I would suggest following:
You bootloader is not locked. Thats fine.
Start in bootloader and do following command:
fastboot flashing get_unlock_ability
Do you get 1 or 0 back?
Be sure that you are in slot a
fastboot --set-active=a