I’m referring to the Operating System…
The newest/latest OS supporting your Mac Pro 1.1 will be OS X 10.7 Lion (latest public release of macOS is 10.14 and 10.15 is just a few months away). OS X 10.7 only support Plex Media Server versions 0.9.16 or below… those releases are 3+ years old!
Server versions that old won’t run with current versions of Plex clients (e.g. on Android / iOS).
