I rarely use nationalcar.com for my overseas rentals as I am supposed to use my companies travel website, but on occasion, I have looked into it.

To answer you last statement, no, your status doesn't really mean anything over there. On occasion I have convinced the agent to give me an upgrade to fullsize (3-series, Passat, C-Class) stating that I am Executive Elite, which is equivilent to their highest status.

You may have no cars showing up because the facility indeed has no vehicles. But, it is most likely due to the EuroCar and NationalCar inventories not speaking. Try booking it through EuroCar.com instead of NationalCar.com.

Also, you cannot use free days in Europe if that is your reasoning for trying to book on NationalCar.com.

Good Luck!

edit - I just realized the Italy part...National does not allow you to take a car larger than midsize I believe into Italy. It could be compact, I don't remember which they made me rent. If you are trying to reserve a higher car class than that, I would assume this is why you are receiving the error.