Add a custom field (in an order) to the emails

wc-add-custom-field.php

/**
 * Add a custom field (in an order) to the emails
 */
add_filter( 'woocommerce_email_order_meta_fields', 'custom_woocommerce_email_order_meta_fields', 10, 3 );

function custom_woocommerce_email_order_meta_fields( $fields, $sent_to_admin, $order ) {
    $fields['meta_key'] = array(
        'label' => __( 'Label' ),
        'value' => get_post_meta( $order->id, 'meta_key', true ),
    );
    return $fields;
}

@marketingsolution the if/else is for emails that are sent in plain text versus emails that are sent with HTML markup. You didn't say what your fatal error was so it's hard to say... though I do see a problem at the end of echo 'The value for <strong>some field</strong> is ' . $NIP.; there's a period just before the semi-colon that should not be there.

function NIP_display_email_order_meta( $order, $sent_to_admin, $plain_text ) {
  $NIP = $order->get_meta( '_NIP' );
  
  if( $plain_text ){ 
	  echo 'Podany NIP to ' . $NIP;
  } else { 
	  echo 'The value for <strong>some field</strong> is ' . $NIP;
  } 

}
add_action('woocommerce_email_customer_details', 'NIP_display_email_order_meta', 30, 3 );