Enable Archive Log

Connect as DBA.

 [oracle@orcl ~]$ sql / as sysdba

Disconnect all users.

Shutdown and startup mount (!Caution).

 SQL> SHTDOWN IMMEDIATE

 Database closed.
 Database dismounted.
 ORACLE instance shut down.

 SQL> STARTUP MOUNT

 ORACLE instance started.

 Total System Global Area 1577055400 bytes
 Fixed Size                  9135272 bytes
 Variable Size             939524096 bytes
 Database Buffers          620756992 bytes
 Redo Buffers                7639040 bytes
 Database mounted.

Enable archivelog.

 SQL> ALTER DATABASE archivelog;

 Database altered.

Open database and start generating logs.

 SQL> ALTER DATABASE open;

 Database altered.

 SQL> ARCHIVE LOG start;
 Statement processed.
 SQL> ALTER SYSTEM switch logfile;

 System altered.

 SQL>