A snap installation of Plex Media Server cannot use hardware accelerated transcoding & tone mapping, as snaps are isolated from the hardware and operating system.
It is possible to transfer a snap installation to a native Linux installation if desired.
Otherwise, you can uninstall the snap version, download the .deb file from plex.tv, and install with “dpkg -i …” (Plex Docs → Installation).
The Plex Linux Tips document is also worth a read. It can help you avoid common pitfalls with installing Plex Media Server on Linux systems.