Missing “Priority”

The recent versions of WooCommerce include a “priority” that indicates the position of the element on the checkout.

On WooCommerce v.3.5.1, this plugin throws a notice.

To resolve, you can edit woocommerce-email-validation/classes/class-woocommerce-email-validation.php and add
'priority' => 110,
(replace 110 with whatever is the correct priority)
under line 49 which is
'validate' => array( 'email' ),