You are working trough ftp client and you can change the file permission from specific file due to a user file creation owner. its usually to see that the file owner/group is different.
Access to Plex server trough SSH client and change the file permission using chmod command.
how to access httpdocs when you are logged in the server trough SSH ?
on the first ls -la you will see something like that:
to open httpdocs folder you have to open: /var/www/vhosts from the home folder