[Solved:]This system is not registered to Red Hat Subscription Management. You can use subscription-manager to register.

Common error when RHEL goes out of subscription/Pool_ID is consumed..

Quick fix for the error is to check anyone/both of the file in the server

/etc/yum/pluginconf.d/subscription-manager.conf
/etc/yum/pluginconf.d/enabled_repos_upload.conf

Content should be:

enabled=0