Code snippet to create order comments field.

woocommerce_create_order_comments_field.php

<?php 

//Create order comments field  

if ( version_compare( WC_VERSION, '3.0', '<' ) ) {
			$fields = WC()->checkout->checkout_fields['billing'];
		} else {
			$fields = WC()->checkout->get_checkout_fields( 'billing' );
		}

  woocommerce_form_field( 
		  'order_comments', 
		  $fields['order_comments'],
		  WC()->checkout->get_value( 'order_comments')
  );