If you run yum update, you may see the following:

[root@rhel-test ~]# yum update
Loaded plugins: product-id, search-disabled-repos, subscription-manager

This system is not registered with an entitlement server. You can use subscription-manager to register.

There are no enabled repos.
 Run "yum repolist all" to see the repos you have.
 To enable Red Hat Subscription Management repositories:
     subscription-manager repos --enable <repo>
 To enable custom repositories:
     yum-config-manager --enable <repo>

Make sure your system is registered. If your system is registered, run the following command:

subscription-manager attach --auto

Wait for the response that the status is "subscribed".  Run yum update again.