Can't find my folder in PMS when browsing for it to add in library

Plex Media Server
2

Make sure the system internal user PlexMediaServer has read (or read/write) access to your media folders.

See these two entries in the Synology FAQ:

Be sure to check the box for Apply to this folder, sub-folders, and files.

FAQ 19: Setting permissions to allow Plex access to media files

FAQ 30 - Migrated to DSM 7 and all my media is gone