I'm guessing this is some sort of specialty vehicle then? Typically for a standard car/SUV/etc, Hertz does not require a CC guarantee. It could just be that Hertz's credit card processing system is down right now. Sometimes you'll get that message if it's a site-wide processor outage. You might just want to try in the morning. If you're still having difficulty, feel free to take a screenshot of the final checkout screen and PM it to us and we'll see what we can do to help you get it booked.