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

alter tablespace users begin backup;

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

Check whether the database in archive log by SYS user

SQL> ARCHIVE LOG LIST;

Database is in non archive mode

We need to change archive mode

SQL>shutdown immediate;

SQL>startup mount;

SQL>alter database archivelog;

SQL>Alter database open;

then you can BEGIN BACKUP command

Cheers!!!