Magento 2 VAT – Enable or Find VAT Registration Number Billing Address

You can try entering this code in class Mage_Customer_Helper_Data:

protected function _createVatNumberValidationSoapClient($trace = false)
{
    $options = array( 'soap_version'=>SOAP_1_1,
        'exceptions'=>true,
        'trace'=>1,
        'cache_wsdl'=> WSDL_CACHE_NONE,
        'user_agent' => 'Mozilla',
        'proxy_port' => 80);
    return new SoapClient(self::VAT_VALIDATION_WSDL_URL, $options);
}

Just to add to webninjas's answer (as it worked for me, I was also missing SOAP), specifically run:

apt install php 7.0-soap

service apache2 restart