I was working on enabling archivelog mode on Oracle Rac 12c database. You can follow below step by step procedure. If you even want to look at disabling archive log mode click on the link( Disable Archivelog Mode Oracle 12c RAC Database).
[oracle@Sajidserver1~]$srvctl status database -d SAJIDDB
Instance SAJIDDB1 is running on node Sajidserver1
Instance SAJIDDB2 is running on node Sajidserver2
[oracle@Sajidserver1 ~]$ sqlplus / as sysdba
SYS@SAJIDDB1>alter system set cluster_database=false scope=spfile sid='*';
System altered.
SYS@SAJIDDB1>exit
[oracle@Sajidserver1 ~]$ srvctl stop database -d SAJIDDB -o immediate
[oracle@Sajidserver1 ~]$ sqlplus / as sysdba
SYS@SAJIDDB1>startup mount;
ORACLE instance started.
Database mounted.
SYS@SAJIDDB1>archive log list;
Database log mode No Archive Mode
Automatic archival Enabled
Archive destination +FLASHDG
Oldest online log sequence 1221
Next log sequence to archive 1222
Current log sequence 1222
SYS@SAJIDDB1>ALTER DATABASE ARCHIVELOG;
Database altered.
SYS@SAJIDDB1>alter system set cluster_database=true scope=spfile sid='*';
System altered.
SYS@SAJIDDB1>shutdown immediate;
ORA-01109: database not open
Database dismounted.
ORACLE instance shut down.
SYS@SAJIDDB1>exit
[oracle@Sajidserver1 ~]$ srvctl start database -d SAJIDDB
[oracle@Sajidserver1 ~]$ sqlplus / as sysdba
SYS@SAJIDDB1>archive log list;
Database log mode Archive Mode
Automatic archival Enabled
Archive destination +FLASHDG
Oldest online log sequence 1228
Current log sequence 1229
SYS@SAJIDDB1>exit
[oracle@Sajidserver1 ~]$ srvctl status database -d SAJIDDB
Instance SAJIDDB1 is running on node Sajidserver1
Instance SAJIDDB2 is running on node Sajidserver2
