Originally Posted by
corporate-wage-slave
If I do an "Inspect" on the field in Chrome, it seems the validation is for a 9 digit VAT number, which is the UK norm, but it's referenced against the Portuguese 7 digit number system. So it's destined to fail. If you change your country to Falkland Islands it then expects 7 digits, and of course the Islands don't have VAT anyway, so that may work for you. Or try another browser which may not validate the number.
Thanks to a tip from another forum I put in my UK national insurance number, and this works fine (this was for a 'personal' payment rather than a business).