I have had a similar problem to some others in that the PMS update on Synology failed and then wouldn't let me reinstall. So following comments on these forums I have updated DSM and I am now trying to remove PMS via SSH so I can reinstall from scratch.....
Following this post from @trumpy81
https://forums.plex.tv/discussion/160972/failed-to-run-the-package-service-synology-nas-ds412#latest
I have removed (logged in as admin via SSH):
/volume1/@appstore/Plex Media Server
/var/packages/Plex Media Server
However when I go to remove Plex from volume1:
"myNas> cd volume1
myNas> ls
@appstore @spool aquota.group
@autoupdate @tmp aquota.user
@database Backup Folder lost+found
@download music
@eaDir Plex photo
@quarantine Synology Nas Shared video
myNas> rm Plex
rm: Plex: is a directory
myNas> rm -rf Plex
rm: can't remove 'Plex': Permission denied
myNas> cd Plex
-sh: cd: can't cd to Plex
what am I missing? removing the other files was no problem
Thanks