Dba.getCluster: This function is not available through a session to a standalone instance (RuntimeError)
It seems the node from the one you are trying to use the command is not part of the Cluster. What you can do is to try to get the status from another node.

If this is not possible, then try to execute:   


  mysql-js> var cluster = dba.rebootClusterFromCompleteOutage();


Anyway, check the follow article: https://mysqlserverteam.com/innodb-cluster-in-opc-part2/