ArchiveLog & enable archivelog mode

ArchiveLog
When you enable this mode redo logs will be archived instead of overwritten.
Archive logs are used by RMAN, Data Guard, Flashback and others.

enable archivelog mode

SQL> SELECT LOG_MODE FROM SYS.V$DATABASE;

LOG_MODE
------------
NOARCHIVELOG

execute below query one by one to enable archivelog mode.
SQL> shu immediate
SQL> startup mount
SQL> alter database archivelog;
SQL> alter database open;
 
SQL> SELECT LOG_MODE FROM SYS.V$DATABASE;

LOG_MODE
------------
ARCHIVELOG