Getting Plex Media Server working under EL7 is an absolute breeze.  Here are the steps needed to get your own server up and rocking.

TO INSTALL

This is getting PMS installed via my yum repository.

yum install https://schotty.com/yum/plex_el7/schotty-plex-el7-release-1-1.noarch.rpm
yum makecache
yum install plexmediaserver

FIX THE FIREWALL TO ALLOW PMS

firewall-cmd --permanent --add-port=32400/tcp
firewall-cmd --reoad

CREATE PLEX SYSTEM GROUP

This is at your discretion as to what to name it, but you will need a dedicated plex group. Ensure that your media folder(s) have group ownership of this media group account.

groupadd media
usermod -G media plex
usermod -G media YOURPRIMARYUSER

ENABLE AND START PMS DAEMON

systemctl enable plexmediaserver
systemctl start plexmediaserver

LOGIN & CONFIGURE

Then in a browser navigate to your server in web browser:

http://IP_OR_DOMAIN_NAME:32400/webindex.html

Then proceed to configure Plex as you see fit. One troubleshooting issue that is worth noting, is that Firefox has an issue with HTML5, so disable that in your web browser if you are having issues.  This is a client side issue, not a server side one.  Chrome and Edge do not currently have this bug.