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.