ORA-38760: This database instance failed to turn on flashback database

 ORA-38760: This database instance failed to turn on flashback database

Solution:
SQL> startup mount;
 SQL> alter database flashback off;
Database altered.
SQL> shutdown immediate;
SQL>  startup
ORACLE instance started.

Total System Global Area 4294967296 bytes
Fixed Size                  3719064 bytes
Variable Size            1308622952 bytes
Database Buffers         2969567232 bytes
Redo Buffers               13058048 bytes
Database mounted.
Database opened.

Delete old file from /flashback/ directory.

 SQL> alter database flashback on;

Database altered.