Assuming you do have Plex Pass, and your environmental variable looks correct, what driver did you install?
Ensure you’re using nvidia’s container toolkit - Overview — NVIDIA Cloud Native Technologies documentation. You can use the toolkit with drivers installed on the host OS, or within containers themselves (doable, but I don’t recommend it) via their native container drivers solution Overview — NVIDIA Cloud Native Technologies documentation
Also, ensure you’re installing the full driver without recommends to avoid the GUI if you’re on a headless machine, but NOT the server version which doesn’t contain all the pieces you need. Since I can only post two links at a time, here’s the code:
sudo apt-get install --no-install-recommends nvidia-driver-435
Grab a newer version of course.
Edit: The container toolkit will allow you to test running nvidia-smi within a container, in case you were only testing above on the host.