I'm guessing that this could have been an error about trying to update from the Software Update CP. I'm even going to say that one of the code developers from Apple was going to pass testing of the final version on to someone else, developing QT6, who would input an appropriate error message in replace of the little statement he was given, and he just forgot about it. It's a guess, but IMBA (In my best assumption) that may be why the message displays that.
Try downloading the entire update directly off Apple.