Please make sure that you have added the country code in front of your VAT ID. For Example: IT0123456789 (please notice the IT in front of the string of numbers)
The system cannot find VAT-IDs that are only numbers, because these numbers can exist in different countries and therefore, you will need to add the country code in front of the VAT - ID.
Here is a list of all country codes.
Please take the Alpha2 codes (2 letters) and add them before the string of numbers.
This issue is also related when you get the message that your VAT – ID is invalid. Add the country code in front and it should work.