Oracle Message RMAN-06132 - cannot backup datafile string because it is not in the control file

ERRORcannot backup datafile string because it is not in the control file
CAUSEA backup command was issued that includes the specified datafile, but the datafile is not listed in the control file. The control file is not current (it is a backup or a created control file).
ACTIONRecover the control file to make it current, then retry the backup command.