While enabling archive mode in database, got below error.

SQL> alter database archivelog;
alter database archivelog
ERROR at line 1:
ORA-01126: database must be mounted in this instance and not open in any instance


The reported error is because changing archive mode need to be done when the database is in mount stage only. If we try to do it in open state, this error will occur.

So to avoid this error, start the database in mount stage and change archive mode.

SQL> select name,open_mode from v$database;

--------- --------------------

SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.

SQL> startup mount;
ORACLE instance started.

Total System Global Area 1.2549E+10 bytes
Fixed Size                 12155024 bytes
Variable Size            6744442736 bytes
Database Buffers         5771362304 bytes
Redo Buffers               21397504 bytes
Database mounted.

SQL> alter database archivelog;

Database altered

SQL> alter database open;

Database altered