Check to see what group /dev/dri/renderD128 belongs to on your system: ls -l /dev/dri
You should see output similar to the following:
drwxr-xr-x 2 root root 80 Feb 27 12:07 by-path
crw-rw----+ 1 root video 226, 0 Feb 27 12:08 card0
crw-rw----+ 1 root render 226, 128 Feb 27 12:07 renderD128
In this case, its group is render. It could be different on your system (it’s video for some, I think). Assuming a default installation, PMS runs as user plex, and needs to be a member of the group shown above. You can check with this from the command-line: groups plex
If plex isn’t in the correct group, add it with usermod: sudo usermod -a -G render plex
These may have defaulted to disabled if you previously did not have a Plex Pass (I’m not sure, it’s been a while for me).
If these are enabled, and you’ve tried restarting PMS and HW accelerated transcoding still doesn’t kick in, you probably need to provide some logs (Debug enabled, Verbose disabled).