When one of your orders has a synchronization error you will receive an email notification about this issue. This is happening when Shoppingfeed sends a new status to a marketplace and gets an error back.
You'll find here the 3 most common reasons, it could happen, and how you can handle it.
1. Missing shipping data - 'Shipping Errors'
For most marketplaces, shipping data are mandatory. You need to provide the tracking number, carrier, and shipping tracking URL. If this information is filled out on your storefront - we will receive it in Shoppingfeed and will then send it to the marketplace. 
So, first of all, you need to make sure that these data are correctly filled out on orders in your storefront. If not, you know that you'll have to fill them for the next orders.
Concerning the orders with this error in Shoppingfeed, you can easily re-send them to the marketplace, by filling shipping data directly from the orders pane.
2. Order Cancellation
API calls for order cancellations are not always available for different marketplaces. So if you have tried to cancel an order and received an error notification, the best way to solve this would be to cancel the order directly from the marketplace Back Office (aka Seller Account).