I have the same problem on my new DS1813+.
Plex did work the first time but then something happen and I uninstalled it and now it doesn´t work.
The install seams to work but then the message "Failed to run the package service" appears.
I have tried to reinstall both the version in synologys database (0.9.7.28.33) and the one on Plex-site (0.9.8.6.175) but both give the same message.
I have also tried to remove the folders
/var/packages/Plex Media Server/target/dsm_config/plex
I have also tried to give the user "Plex" R/W-access to all folders...
The package is installed on volume1...
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