Every time ApiDrop imports your orders also checks that products listed are present in your store.
When a product is present inside the order but is not present in your "My Store" list, order import will raise an exception named "Missing order items"
How to solve: Take note of missing item(s) sku(s) and from products catalog page, add them to your store.
Once the products are back in sync and their sync status is "Done", return on the error page and click on the "Retry Import" button.
If everything is ok, order will be imported without problems.