never mind.
I read this post (Plex can't see folders)
usermod -a -G users plex and a service plexmediaserver restart after that.
it resolve my issue
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I read this guide from smokingplatypus, wonderful writeup:
Install of PlexMediaServer Plugin
my music folder structure is homeland/Media/Music
I checked the acl permission has "other = read only"
I stuck at step 13 "(web GUI) Services > Plex Media Server > Settings (tab) > select the Plex Web Client (button) > a new tab/webpage will open to the Plex Media Server interface "
in Plex Web Client, when I attempt to add the library, it doesn't allow me to select "Media" or "Music".
if I didn't choose the subfolder, just create add, it allow me to create the library, but scan find no music. but i have some mp3 file there.
I wonder if it's permission thing?
Many thanks!