I happen to have a fresh Silverblue install here and the systemd check at the beginning of the preinstall script is failing which doesn’t immediately make sense to me because cat /proc/1/comm returns systemd. This check will also fail for toolbox environments. This is a fixable thing in the RPM spec, if it were a Fedora packaged thing, but unfortunately, you’re at the mercy of Plex for this one.
A quick look through past discussions on this has shown that others ended up running plex server in podman.