Validating custom checkout fields in admin edit order form - php

Question

I'm trying to add validation to some checkout fields when editing an order from admin panel, but I haven't found the hook for that. I want to prevent saving post if validation fails and also show a notice. I've tried woocommerce_process_shop_order_meta hook but post is always saved and notices are not shown.
This is the code I used in my functions.php file:
$meta_box_errors[] = array();
add_action( 'woocommerce_process_shop_order_meta', 'save_order_custom_field_meta_data', 12, 2 );
public function save_order_custom_field_meta_data( $post_id, $post ){
$field_name = 'billing_nif';
$nif_value = '';
if( isset( $_POST[ '_' . $field_name ] ) && !empty($_POST[ '_' . $field_name ]) ) {
$nif_value = sanitize_text_field( $_POST[ '_' . $field_name ] );
if (!empty ($nif_value)) {
if (! validate_nif( $nif_value )) {
$meta_box_errors["invalid_nif"] = 'Invalid NIF';
}
} else {
$meta_box_errors["empty_nif"] = 'Empty NIF';
}
} else {
$meta_box_errors["empty_nif"] = 'Empty NIF';
}
if( ! empty( $this->meta_box_errors ) ) {
add_action( 'admin_notices', 'output_errors' );
}
}
public function validate_nif( $nif ){
$nif = strtoupper($nif);
if (strlen($nif) != 9) {
return false;
}
$nifExp = "^((\d{8})([A-Z]{1}))$^";
return preg_match($nifExp, $nif));
}
public function output_errors() {
echo '<div id="woocommerce_errors" class="error notice is-dismissible">';
foreach ( $meta_box_errors as $error ) {
echo '<p>' . wp_kses_post( $error ) . '</p>';
}
echo '</div>';
unset($meta_box_errors);
}