You should install the latest Android Platform Tools:
Currently v33.0.3
Old versions of platform tools will cause fastboot commands to FAIL!!
Make sure you are in fastbootd mode as well:
Fastbootd is the second stage loader, which can handle the dynamic partitions.
it is reachable by the very poorly named command
fastboot reboot fastboot
(which should have been named fastboot reboot fastbootd).
It reboots to a special version of the Recovery mode, running Fastbootd.
It is a black screen with the one plus logo and the word "fastbootd" at the top, and some touch menus.