Systemctl start mattermost.service is failing

Well, after some more searching, it appears that turning off or making permissive SELinux fixed the issue.

I disabled SELinux by editing /etc/selinux/config and changing the entry:
SELINUX=enforcing
to
SELINUX=disabled

It worked before this changing the environment with:
setenforce 0

This changed SELinux to permissive.

However, I went ahead and disabled it and made it persistent by also editing file (above).