ORA-38706: Cannot turn on FLASHBACK DATABASE logging

SQL> alter database archivelog;

Database altered.

SQL> alter database flashback on;
alter database flashback on
*
ERROR at line 1:
ORA-38706: Cannot turn on FLASHBACK DATABASE logging.
ORA-38709: Recovery Area is not enabled.
 

SQL> select log_mode from v$database;

LOG_MODE
------------
ARCHIVELOG

SQL> alter system set db_recovery_file_dest='/u02/flash_back' scope=both;
alter system set db_recovery_file_dest='/u02/flash_back' scope=both
*
ERROR at line 1:
ORA-02097: parameter cannot be modified because specified value is invalid
ORA-19802: cannot use DB_RECOVERY_FILE_DEST without DB_RECOVERY_FILE_DEST_SIZE

SQL> alter system set db_recovery_file_dest_size=2g scope=both;

System altered.

SQL> alter system set db_recovery_file_dest='/u02/flash_back' scope=both;

System altered.

SQL> alter database flashback on;

Database altered.