Tax validation in Craft Commerce
Probably best to just update the address business tax ID if it is in the wrong format automatically for them.
Event::on(Order::class, Order::EVENT_BEFORE_SAVE, function(\craft\events\ModelEvent $event) {
    /** @var Order $order */
    $order = $event-&gt;sender;
    if ($order-&gt;getShippingAddress() &amp;&amp; $order-&gt;getShippingAddress()-&gt;businessTaxId) {
        $newTaxId = &quot;AU&quot;.$order-&gt;shippingAddress-&gt;businessTaxId; // transform this into the right format based on address country (if it is not already)
        $address = $order-&gt;shippingAddress;
        $address-&gt;businessTaxId = $newTaxId;
        $order-&gt;setShippingAddress($address); //  address will save when order saves.
    }
});