I think this addon is only for x86, not for arm (pi).
You can use pi as Plex server with other os, like dietpi.
Edited: You can install docker on LibreELEC. Then run plex from docker container instead. (See this repo)
- after first install, wait for 5 minutes (or more maybe) before try to access plex web interface.
Linuxserver.io repo is now in the libreelec repo. Once you install from there, then you can install the docker addons including plex media server from the linuxserver repo