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

Cause:

Unexpected server reboot.

When starting the instance after mount we will face this issue.
This is due to missing current flashback log missing/deleted

SQL> startup
ORACLE instance started.
Total System Global Area 1161966444 bytes
Fixed Size 1787966 bytes
Variable Size 1145750568 bytes
Database Buffers 225163524 bytes
Redo Buffers 1262132 bytes
Database mounted.
ORA-38760: This database instance failed to turn on flashback database.

Fix:
Solution A:

1. startup mount;
2. alter database flashback off;
3. alter database flashback on;
4. alter database open;

Solution B:

1. Startup mount.
2. Check db_recovery_file_dest parameter and ensure this directory exists.
3. Check restore points:
select NAME,SCN,GUARANTEE_FLASHBACK_DATABASE,DATABASE_INCARNATION#
from v$restore_point;
4. Drop restore point:
drop restore point ;
5. alter database flashback off;
6. Shutdown immediate;
7. Startup