This customer's location couldn't be validated as required by the new EU VAT 2015 rules
Step 1: Locate such customers.
You could filter the customer whose address is not validated by using the filter "Is location valid" "is not" "true".
This filter would give you the list of all customers whose location is not validated according to EU VAT rules.
When you open the customer record you would see a banner as in the screenshot below.
Step 2: If you are using API so you would have to pass the data for verification using API.
Customer's IP Address:
API: If you use the Chargebee API, you have to pass the IP Address of the customer to Chargebee using the User Details Header API.
Card BIN of the Customer:
The first six digits of a card carry the Bank Identification Number (BIN). BIN gives us information about the card-issuing bank and hence can be used as a way to determine the customer's location.
API: If you use the Chargebee API, you have to pass the BIN of the customer using the card [number] parameter in the Create a customer API.
Step 3: Check if all the customers who were not imported but created via API have their validation status as false.
You could send them the updated payment method link and ask them to update their card from their corresponding countries which would then update the IP and the BIN numbers.
Refer to this link for more information.