How to add a library

[tom80H]

That sounds like a permission issue.
You might want to check out the Plex for Syno FAQ for details on how to configure your NAS to work with Plex.

Synology FAQ - Questions, Answers, and a few How-To's

Synology Frequently Asked Questions Table of Contents Intro: Posting in the Plex Synology Forum DSM 6 & 7 FAQ Some of these FAQ apply equally to DSM 7 and 6. Others do not. Please read carefully before proceeding. Getting access to the Logs Have I installed the latest version…? Which version do I need, Intel or ARM? 3a. Using Plex’s package signing public key - DSM 6 LAN addressing requirements - RFC1918 specification for local LAN? First run & Setup Recommended Media Organization and A…