Check if ambari-agent is installed on your data nodes if not?
# yum install ambari-agent
If your repo is correctly set /etc/yum.repos.d/hdp.repo etc then it should pick the correct agent, after the installation
Make sure the ambari-agent.ini has the correct ambari-server FQDN the start the agent
# ambari-agent start
After a few minutes, it should start reporting the heartbeat to the ambari server UI.
That looks like the status from the Ambari server, not namenode. Can you click on the cog wheel of hdfs client or the triangle extreme right from there you can drill down?