RedHat sells RHN subscriptions. It's how they make money. Because you've got more than one system registered, and you only get one freebie, it is now complaining that your second system, which you apparently haven't paid for, isn't entitled. If you don't use your previous system on RHN anymore, login at http://rhn.redhat.com/
and swap your entitlements around, following exactly the instructions contained in the above error message.