It requires regulated 12v and your vehicle's electrical system doesn't provide regulated 12v.
Can you use the board? Sure, with a regulator.
I was looking at a Dual core Atom + Ion combo as a carputer, and I noticed it came with a 40w onboard DC-DC PSU. Does this mean I can't set it up for a car?
slight correction-- there is no link to the product your talking about, so i can't be sure that it has a on-board 12 volt dc-dc. many atom boards have a 19v power adapter, which would have different in-car power supply requirements.
saying it uses dc-dc power is like asking why your Ford won't start-- there are many variations within the category, and all of them can have different solutions.
doesn't say the input voltage on the details page.
you need atleast 9-16. or you will probably burn the dc-dc converter when you start te car
(car running is13.8 volts or higher)
Car system in the making:
M2 atx psu
the faq's cover how to pick the correct power supply (link in darques sig)