They probably have LOR restrictions in. (Length of Rental). Since you are renting mid-week (where there is peak demand) and keeping it a short time, Hertz sometimes won't take reservations for 1-2 day rentals. Try 3 days or longer. It's more cost effective to turn down a one day rental and take a three day rental because you don't have to rent the car, check it in, service it, etc. as often.

We had this happen all the time at the location I used to manage. Just book for 3 days and return after one day. Remember that many corporate rates have a slight surcharge for renting 1-2 days midweek (usually $5 a day) so don't be surprised if your rates edges up slightly upon return. If you were able to book it for one day in the beginning that rate would have been provided.