I just got my Synology DS218, with DSM 6.2.1-23824 Update 1.
I have been trying to install PLEX version PlexMediaServer-1.13.8.5395-10d48da0d-x86_64,
however, getting error message: This package is not supported on the platform of DiskStation or is incompatible with the current DSM Version.
I have looked some other people have no issue with different type of DS machine, with the same version of DSM and Plex server.
I downloaded the 64bit package from Plex.TV today (11/20/2018) and it appears to have installed (v.1.14.0.1465). Can’t figure out how to run it from the NAS, but it’s not showing any errors (green Running status) on the server itself.