It's uncommon to find a Minivan in UC in general. I would think it would be highly unlikely to find one if the location is sold out. A few suggestions:
1. Keep searching each day. Sometimes availability opens up, often for a short period, and you may be able to catch something if you are diligent.
2. Check Fullsize SUVs and some of the specialty vehicles that hold 7+ passengers. You may find something else that meets your needs.
3. Obviously, check other companies. I know you are Hertz PC, and would prefer renting with them, but always good to have a backup in case that doesn't meet your needs.