<- DataGuard

Enable Flashback Database

 -- On Primary 
 sqlplus / as sysdba

 -- On SB
 dgmgrl sys/go@oradb_sb
 DGMGRL> edit database oradb_sb set state=apply-off;
 DGMGRL> sql "alter database flashback on";
 DGMGRL> edit database oradb_sb set state=apply-on;

 -- On Both (QC check)
 SQLPlus> SELECT flashback_on FROM v$database;

The standby redo apply services must be stopped on the Standby before flashback database can be enabled.

If Flashback is not enabled, you will have to manually recreate your Primary as a Standby in a failover event to reinstate it.

With multi-terabyte databases common and depending on the network connectivity between your primary and standby sites, this could end up being quite a lengthy process.