While subscribing system to a sub-channel or a child channel using "rhn-channel" command, why it fails with an error "Error Class Code: 40"?

Issue

• Subscribing a system to a child channel fails with error below:

[root@mysystem ~]# rhn-channel --add --channel=rhel-x86_64-server-dts-6

Error communicating with server. The message was:

Error Message:
    Channel rhel-x86_64-server-dts-6 does not exist?
Error Class Code: 40
Error Class Info: The specified channel does not exist.
Explanation:
     An error has occurred while processing your request. If this problem
     persists please enter a bug report at bugzilla.redhat.com.
     If you choose to submit the bug report, please be sure to include
     details of what you were trying to do when this error occurred and
     details on how to reproduce this problem.