Enable flash recovery area .

1.First set up the database in archive log mode.
2.Next set up the flash back
SQL> show parameter recovery

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_recovery_file_dest string G:\oracle\product\10.2.0/flash
_recovery_area
db_recovery_file_dest_size big integer 2G
recovery_parallelism integer 0
SQL> select FLASHBACK_ON from v$database;

FLASHBACK_ON
------------------
NO

3.Shutdown the database and mount the database ,And enable flash back on database.
===================================================================================
SQL> startup mount;
ORACLE instance started.

Total System Global Area 574619648 bytes
Fixed Size 1250212 bytes
Variable Size 171969628 bytes
Database Buffers 394264576 bytes
Redo Buffers 7135232 bytes
Database mounted.
SQL> alter database flashback on;

Database altered.

SQL> alter database open;

Database altered.

SQL> select FLASHBACK_ON from v$database;

FLASHBACK_ON
------------------
YES