When adding your VAT number to the settings page you may receive an error that says: "Incorrect tax registration number format, update or leave blank"
Then check the following:
- The VAT number should have the two country code letters at the start.
- The VAT number should not contain any spaces or hyphens.
- If it still does not work test the number here: http://ec.europa.eu/taxation_customs/vies/. This is the European database of VAT numbers.
- If it does not recognise your number then you need to contact your country office responsible for VAT numbers. They can make sure your number is recognised. See the contact details here: http://ec.europa.eu/taxation_customs/vies/faq.html#item_7