You have to create a recovery catalog first.
Create a new database or pickup an existing database.
Create a tablespace for RMAN objects, about 50 MB size.
Create a user 'rman' and set his default tablespace as that created above.
Grant RECOVERY_CATALOG_OWNER to rman.
Grant DBA, Connect & Resource roles to rman.
Now connect to recovery catalog as user rman to create recovery catalog objects.