ORA-01123: cannot start online backup; media recovery not enabled

SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount
ORACLE instance started.

Total System Global Area 1310284300 bytes
Fixed Size 457228 bytes
Variable Size 234881024 bytes
Database Buffers 1073741824 bytes
Redo Buffers 1204224 bytes
Database mounted.
SQL> alter database archivelog;

Database altered.

SQL> alter database open;

Database altered.

SQL> archive log list
Database log mode Archive Mode
Automatic archival Enabled
Archive destination /disk01/archive/xxxxx
Oldest online log sequence 84
Next log sequence to archive 86
Current log sequence 86