Some US banks do not allow international transactions by default. Since Driving-Tests.org is headquartered in Ontario, Canada, your card may be declined because your bank sees us as an "international merchant".
There are a few things you can do to try and fix this:
Talk to your bank and ask them to enable international transactions on your card.
Use a different credit or debit card.
Purchase a prepaid credit card and use that to sign up for a Driving-Tests.org Premium account.