Validating Custom Fields

functions.php

<?php
add_action( 'wp_enqueue_scripts', 'child_enqueue_styles' );
function child_enqueue_styles() {
    wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' );

}

add_action('woocommerce_after_checkout_validation', 'digthis_two_step_checkout_validate', 9999, 2);
function digthis_two_step_checkout_validate($data, $errors) {
	
	$step = filter_input(INPUT_POST, 'current_step');
	
	if( empty( $errors->errors ) && $step == 'step-1'  ){
		$errors->add( 'digthis', __( '<span id="digthis-prevent-error">Digthis Error</span>', 'woocommerce' ) );
	}

}

add_action('woocommerce_before_checkout_billing_form', 'customise_checkout_category_user', 0);

function customise_checkout_category_user($checkout) {

	woocommerce_form_field( 'inscription_textbox_cf', array(
	'type' => 'text',
	'class' => array( 'inscription-text_cf form-row-wide' ),
	'label' => __( 'Codice Fiscale' ),
	'required' => true,
	), $checkout->get_value( 'inscription_textbox_cf' ) );

	woocommerce_form_field( 'inscription_textbox_iva', array(
	'type' => 'text',
	'class' => array( 'inscription-text_iva form-row form-row-last' ),
	'label' => __( 'Partita Iva' ),
	'required' => true,
	), $checkout->get_value( 'inscription_textbox_iva' ) );

}

add_action('woocommerce_after_checkout_validation', 'validate_checkout_category_user', 10, 2);
function validate_checkout_category_user($data, $errors) {
	
	$inscription_cf = filter_input(INPUT_POST, 'inscription_textbox_cf');
	$inscription_iva = filter_input(INPUT_POST, 'inscription_textbox_cf');
	
	if( $inscription_cf != 'digthis' ){
		$errors->add( 'digthis', __( 'value should be digthis', 'woocommerce' ) );	
	}

	if( $inscription_iva != 'checkthis' ){
		$errors->add( 'digthis', __( 'value should be checkthis', 'woocommerce' ) );	
	}
}