Free Day(s) can only be used on National Car Rental Brand reservation requests at participating National
Car brand rental locations in the U.S., Canada, Australia, the Latin American Caribbean
(and, in Europe, for National Car rental brand reservations serviced by Enterprise).
I just spent a while on the phone - and they peg the inability to rent in Italy on the "participating" word.
They also told me on the phone that there are "currently no participating locations" in Europe, Australia, or the Caribbean. (!)