ORA-38729: Not enough flashback database log data to do FLASHBACK.

Just check the oldest flashback time available>

select oldest_flashback_time from v$flashback_database_log;

Cause:

We need to have the required archive logs in the archive destination at the time of flashback.

Like, there could be a  backup script is archiving all the archivelogs and purging it from the ASM, hence, we cannot flashback with the error that it cannot locate the log files.
 
Resolution:
 
That is, if the flashback database command does not work in SQLPlus because a needed archive log was deleted after an RMAN backup, execute the flashback database command from RMAN. RMAN will flashback the database, restore the missing archive log(s) and apply the logs as needed.

Then 'alter database open resetlogs ;' is executed after the flashback database as normal.

 

Ref: https://support.oracle.com Document ID 560686.1.