This error appears when an address being used does not have a VAT Registration Number. It may be that one of the addresses on an account does not have a VAT Registration Number. To check this please follow the steps below:
Go to the Creditors or Debtors tab (depending on if this relates to a supplier or customer)
Click Suppliers or Customers
Click the account with the affected address
Click the Address button in the record section
Select the affected address and click the edit button
Click the VAT button in the record section
Enter the required VAT Registration Number
Click save
After this has been filled out and saved, you should now be able to use the address as expected.