According to that article, you can check if you have access to Red Hat Software Collections (RHSCL) by running the following command by the root user:

$ su -
# subscription-manager repos --list | egrep rhscl

If you have, enable necessary software repo and then install devtoolset:

# subscription-manager repos --enable rhel-7-server-optional-rpms
# yum install devtoolset-8

Try:

yum group list

To see if yum can find the Development Tools under Available Groups. If it fails, try:

yum clean all
yum group list

And see if it can find it. If it still fails, you might not have the correct mirrors enabled to resolve it.

try the clean command again and then:

yum --disablerepo=\* --enablerepo=base,updates group list

If you can find it. Great! Install with:

yum --disablerepo=\* --enablerepo=base,updates groupinstall "Development Tools"

If not so, can you give us the output of:

yum repolist all

EDIT:

Thank you for the output.

I found an article which state the following: "To install the CentOS Development tools, you will need to be able to connect to your Virtual Server using SSH, and work as the root user."

Try switching to user "root" (if you don't know the root login credentials, you can try sudo su. sudo also might work but I see you already have tried that...) and running the above commands.

Lastly, you can try and enable these "sources" repos:

• CentOS-7 - Base Sources
• CentOS-7 - Extras Sources
• CentOS-7 - Updates Sources

And than running yum update followed by groupinstall.

Let me know if that doesn't work. Good luck!