Why am I unable to attach a subscription to a manually created system in the Customer Portal and getting 'Too many content sets for certificate' error?

Issue

• Why am I unable to attach a subscription to a manually created system in the Customer Portal and getting 'Too many content sets for certificate' error?

Sample error:

Too many content sets for certificate Red Hat Enterprise Linux Server, Standard (1-2 sockets) (Up to 1 guest). A newer client may be available to address this problem. See kbase https://access.redhat.com/knowledge/node/129003 for more informatio

Note: This information is for Registering an Offline System, 5.1. Registering a New System of Red Hat Network Subscription Management Guide.