Plex cannot see anything in or under the users home directory by default due to permissions/
This is the default that you will probably see
$ ls -l /home
total 28
drwxr-x---. 100 USER USER 12288 Oct 29 15:12 USER
If you use the command sudo chmod o+rx /home/USER it changes to
$ ls -l /home
total 28
drwxr-xr-x. 100 USER USER 12288 Oct 29 15:12 USER
and plex should then be able to find the video library at /home/USER/Movies/*.
I ran into the same issue when I first began using plex for my home video & music server, and is a result of the linux permissions for access to files & directories.
I wouldn’t recommend doing this, since it gives all users that have access to this machine access to your home folder.
You might be able to give just the plex user / group access … but I usually solve this in a different way - storing Media files in a separate location that’s outside my $HOME directory, and giving “other users” access to those external folders only.