Cause: The file access enqueue could not be obtained for a file specified in a backup, copy or restore operation. If the enqueue type shown is "shared", then the file is the input file for a backup or copy. If the type is "exclusive", then the file is the output file for a datafile copy or restore which is attempting to overwrite the currently active version of that file - in this case, the file must be offline or the database must be closed. If the type is "read-only", then you are attempting to back up or copy this file while the database is in NOARCHIVELOG mode.