If a database runs in noarchivelog mode, then the only valid database backup is a consistent backup. So we need perform a clean shutdown first, then start the database in mount status.

1 Shutdonw the database consistently and then mount it.

shutdown immediate;
startup force DBA;
shutdown immediate;
startup mount;

2 connect the target database with rman and run the backup database command.
RMAN> backup database;
RMAN> backup as copy database;


3 open the database;
alter database open;