ORA-38754: FLASHBACK DATABASE not started; required redo log is not available

SQL> flashback database to restore point B4TEST;
flashback database to restore point B4TEST
*
ERROR at line 1:
ORA-38754: FLASHBACK DATABASE not started; required redo log is not available
ORA-38762: redo logs needed for SCN 5974114096519 to SCN 5974114096614
ORA-38761: redo log sequence 165 in thread 1, incarnation 1 could not be
accessed

The flashback rollback database by using flashback logs, then it recovers to desire point in time using archived log files. You need to have the required archive logs in the archive destination at the time of flashback then re-issue the flashback to restore point again.