To supporting SI CAPTCHA on WooCommerce registration form place below code in functions.php

// Let’s try some anti-spam on user registration
add_filter( ‘woocommerce_process_registration_errors’, ‘op_woocommerce_sicaptcha_registration_filter’ );
function op_woocommerce_sicaptcha_registration_filter($errors) {
if (class_exists(‘siCaptcha’)) {
$si_image_captcha = new siCaptcha();
$errors = $si_image_captcha->si_captcha_register_post($errors);
}
return($errors);
}