Verify that your environment meets the system requirements for Always On availability groups.
If the replicas are not Synchronized or Synchronizing, see Article ID: SQL0018.
If you have a restricted network configuration, you will need to verify that the communication is open to the ports that are used by the Commvault software on all the AG clients. The AG clients will not only need communication open to the CommServe and the MediaAgents, but the AG clients themselves will need communication open for the software on each node to be able to communicate with each other during backup. The AG client communication that is open to each other is required to help determine which replicas to perform backups based on the configuration.