If I have a value (even a totally valid one as shown below, though that was not my initial test) in my environment for PBS_SERVER then it is missing from pbs.conf. I see this for pbspro-server, pbspro-execution, and pbspro-client (not even the "CHANGE_THIS_TO_PBS_PRO_SERVER_HOSTNAME" value is present when using those last 2).
[root@centos7 pbspro-14.1.0]# export PBS_SERVER=centos7
[root@centos7 CentOS_7]# rpm -i pbspro-execution-14.1.0-13.1.x86_64.rpm
warning: pbspro-execution-14.1.0-13.1.x86_64.rpm: Header V3 RSA/SHA256 Signature, key ID b5eddb83: NOKEY
PBS Installation Summary
Postinstall script called as follows:
/opt/pbs/libexec/pbs_postinstall execution 14.1.0 /opt/pbs /var/spool/pbs
No configuration file found.
Creating new configuration file: /etc/pbs.conf
Replacing /etc/pbs.conf with /etc/pbs.conf.14.1.0
/etc/pbs.conf has been created.
Registering PBS Pro as a service.
PBS_HOME is /var/spool/pbs
Creating new file /var/spool/pbs/pbs_environment
WARNING: TZ not set in /var/spool/pbs/pbs_environment
The PBS Pro MOM has been installed in /opt/pbs/sbin.
The PBS commands have been installed in /opt/pbs/bin.
End of /opt/pbs/libexec/pbs_postinstall
[root@centos7 CentOS_7]# cat /etc/pbs.conf
PBS_SERVER must not be missing in pbs.conf