Kevin,
Here are the versions:
DNN Version: 06.01.02 (98)
SmithCart version: 4.87.0
I'm using Elavon Virtual Merchant as the gateway.
I see that the developer's guide lists it as not required, but I also see that other cart providers seem to have the same issue with this field being missing. I can't tell if the field should just be the customer number from the cart system (I think that would work) or something else.
Strangely it doesn't appear that it is always a problem. I have a few customers who always get the error (tried multiple times with multiple cards), but I have had two customers that did successfully purchase (and never saw the error at all). I'm not certain that there's a correlation (not enough purchases yet to say it's significant), but it appears that the failures are always with non-US based customers.
I just verified your super-user account is still set up.