When entering the travellers passport details when booking on behalf of a traveller the system asks for a country code. Upon entering the country code a "booking failed due to incorrect country code" notification appeared. It seems for US travellers neither US or USA works when inputting the code. 

I have also attempted not to include a country code as it does say that it is optional, however, this did not work either. 

Please advise me on why this notification keeps appearing and what should be entered as the country code for US travellers. 

Thank you!