While putting the database in hot backup mode  , below error came.

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

PROBLEM:

SQL> alter database begin backup;
alter database begin backup
*
ERROR at line 1:
ORA-01123: cannot start online backup; media recovery not enabled

SOLUTION:

This error comes, when the database is in noarchivelog mode;

SQL> archive log list;
Database log mode              No Archive Mode
Automatic archival             Disabled
Archive destination            +B2CARCH02/b2ccrmpd/arch
Oldest online log sequence     2151
Current log sequence           2156

Enable the archive log mode and run begin backup command.

shutdown immediate
startup mount
alter database archivelog;
startup force;

Now run begin backup:

SQL> alter database begin backup;

Database altered.