It's saying your RedHat Network (RHN) license is not valid. So you can't download software from repositories.
Try using CentOS instead of RedHat. You can convert RedHat 5 to CentOS 5
or you can convert RedHat 6 to CentOS 6
. Your other alternative is to purchase a RedHat license.
edit: I just realized you said you were using rhel6 so just use that appropriate article I listed above. Also for others who don't know what RedHat version you're using you could...