Message Oracle NNL-00005 - no server has been set. Use the SET SERVER command first

ERRORno server has been set. Use the SET SERVER command first
CAUSEThe user attempted to issue a command which requires that a default managed Oracle Names server be set first.
ACTIONUse the SET SERVER command to set your default server, then reissue the command. Alternatively, if the command allows, re-issue it with a specific server name so the control program does not need to use the default.