Lastest Plex Server Update Not Compatable with Synology DS218+ DSM

That’s correct.

Per my announcement Synology installation changes for Plex Media Server

DSM 6.2 Update 2 no longer permits 32 bit packages (x86) on 64 bit processors . You must manually download the 64-bit version from plex.tv/downloads and install per the instructions above.

This is well documented throughout the forums.