After installing a primary under a central admin site you may get the following message when attempting to open the console at the primary site level: "Your Configuration Manager console is in read-only mode while this site completes tasks related to maintenance mode". This usually indicates that the initial replication between the CAS and the primary isn't complete yet. Monitor rcmctrl.log on the CAS and also the Database Replication view in the Monitoring workspace on the CAS. Once the link state says 'Link is active' for the child sites then the replication should be complete and you should be able to open the console at the primary without getting the read-only message. Also note that this replication may take some time. My environment took at least an hour to complete.