Read the Synology FAQ.
Opening input file: "/volume1/Plex/Movies/Motherless.Brooklyn.2019.1080p.BluRay.x264-SPARKS[rarbg]/Motherless.Brooklyn.2019.1080p.BluRay.x264-SPARKS.mkv"
You have your media installed in the wrong location. Do not install media in /Plex. See FAQ 6.
After re-organizing your media, check permissions.
See FAQ: Setting permissions to allow Plex access to media files
Although not strictly required, suggest you follow Plex media naming guidelines. Plex does a better job of matching, downloading metadata, etc when you follow the guidelines. This is especially true for TV shows, with which Plex is very picky.
Instead of:
/Movies/Motherless.Brooklyn.2019.1080p.BluRay.x264-SPARKS[rarbg]/Motherless.Brooklyn.2019.1080p.BluRay.x264-SPARKS.mkv
Use:
/Movies/Motherless Brooklyn (2019)/Motherless Brooklyn (2019).mkv
Alternately, use square brackets to hold the non-Plex info. Plex ignores information between the first set of square brackets.
/Movies/Motherless Brooklyn (2019) [1080p.BluRay.x264-SPARKS]/Motherless Brooklyn (2019) [1080p.BluRay.x264-SPARKS].mkv
