ORA-01123: Unable to start online backup; Media recovery not enabled (Error analysis)

1. Online Backup error:
sql> alter tablespace ZY begin backup;
Alter TABLESPACE users begin backup
*
ERROR at line 1:
Ora-01123:cannot Start online backup;media recovery not enabled 2. Find the cause of the error:
Alter TABLESPACE XXX begin backup cannot be done in noarchive mode, and online backups can only be made in the Archivelog mode in which the database is located. 3. Look at the archiving situation:
Sql> Archive Log List
Database log mode No Archive mode
Automatic Archival Disabled
Archive Destination Use_db_recovery_file_dest
Oldest online log sequence 36
Current log Sequence 38 4. Start the database in archive mode:
Sql> Shutdown ImmediateSql> Startup MountSql> ALTER DATABASE archivelog; 5. View the archive:
Sql> Archive Log List
Database Log Mode Archive mode
Automatic Archival Enabled
Archive Destination Use_db_recovery_file_dest
Oldest online log sequence 37
Next log sequence to archive 39
Current log Sequence 39 6. Open the database
Sql> ALTER DATABASE open;
Database altered. 7. Turn on online backup mode
Sql> Alter TablespaceZy Begin Backup;
Tablespace altered.