To switch on flashback on the database if FRA space is not sufficient to accomodate the flashback log files then,
SQL> alter database flashback on;
alter database flashback on
*
ERROR at line 1:
ORA-38706: Cannot turn on FLASHBACK DATABASE logging.
ORA-38708: not enough space for first flashback database log file
SQL> !oerr ora 38708
38708, 00000, "not enough space for first flashback database log file"
// *Cause: An ALTER DATABASE FLASHBACK ON command failed because there
// was not enough space in the Recovery Area for the first
// flashback database log file.
// *Action: Make more space in the Recovery Area. For example, this
// can be done by increasing the value of
// DB_RECOVERY_FILE_DEST_SIZE.
Increase FRA!
SQL> alter system set db_recovery_file_dest_size=10000M;
System altered.
SQL> alter database flashback on;
Database altered.
