Also, try doing manually in the UI what your integration is doing. I have a vague recollection from years ago that there used to be two different tax fields, one for US-based customers and one for non-US-based customers. I think Vat Reg Num is the one for non-US. Depending on the customer, it would only show one field or the other. So if the field isn't visible/available because it's a US customer, that could be why you're getting that error.
Note that this might have been changed like 5 years ago and I just haven't noticed...lol. But just throwing out shots in the dark.