ORA-00951: cluster not empty
Cause: A DROP CLUSTER statement specified a cluster that is not empty. A cluster may not be dropped if it contains any tables, unless the optional INCLUDING TABLES clause is specified. Tables may also be removed from a cluster by using the DROP TABLE command.
Action: Either specify the INCLUDING TABLES clause in the DROP CLUSTER statement or remove all tables from the cluster with the DROP TABLE command before issuing the DROP CLUSTER command.