Hi,
This indicates that at least one of the addreses of the user account you're logged in with in your backend has an invalid VAT number.
So I would recommend you to check the addreses of your user via the Customers menu.
Also, we'll add the code:
hikashop_nocache();
hikashop_cleanBuffers();
before the line:
echo $newprice;
in the file administrator/components/com_hikashop/controllers/product.php
That will avoid displaying the error while you're editing / adding a price as that's not the place for that