Create a tablespace e.g. RMANTBS (RMAN alone is reserved word)
Create a user RMAN
Grant the user with connect,resource and recovery_catalog_owner
Then into the RMAN on catalog instance (RMAN catalog rman/password@connection)
create catalog tablespace RMANTBS (don´t use dots or any sign)
Then into the RMAN on target instance
rman catalog rman/password@cataloginstance target rman/rman@targetinstance
register database
And if you deleted some archives then
crosscheck archivelog all (to syncronize it)
backup database plus archivelog (make a complete set of backup, including redos generated during backup)
