XWindows didn’t get installed on your RHEL 8/7 (or clone centos etc). Then do this

BTW if XWindows is not loaded on your linux app for RHEL or clones you can go here, download the RPM and do a yum install like this:

https://rpmfind.net/linux/rpm2html/search.php?query=xorg-x11-apps&submit=Search+...&system=&arch=

$ yum install org-x11-apps-7.7-21.el8.x86_64.rpm
# or
sudo yum install xorg-x11-apps

Another oddity – with Centos8 Gnome DISPLAY is set like this: export DISPLAY=:0

In other words normally for xWindows you do something like this to set your DISPLAY variable prior to running an xApp like:

export DISPLAY=<machine-name>:0.0

With Gnome the syntax is:

export DISPLAY=:0

All other common version of setting display do not work on Gnome.

One other tip – it appears RHEL 8 no longer allows root account to “xhost +” but other accounts can.

See this for more info:

http://www.teamhealthgroup.net/index.php/2021/09/24/xhost-has-changed-with-rhel-8-xhost-many-times-returns-a-cannot-open-display-try-this-instead/