When one of your orders has a synchronization error (wrong status) 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 in return from the marketplace.
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 of the marketplaces, shipping data are mandatory. You need to fill in the tracking number, carrier, and shipping tracking URL and If this information is filled out on your storefront, we will receive it and Shoppingfeed will 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 order pane.
2. Invalid status
Sometimes the error comes from the status that Shoppingfeed is trying to send.
It's not possible to update order status from one to another IF the previous status was not correct. For example, it is not possible for an order to be updated as "SHIPPED" if the previous status was "Processing" instead of "Accepted".
In order to solve this issue, you should check the status on the marketplace Back Office. If the status is not Accepted (already shipped, cancelled, unverified), please contact us, and give us the order number, the correct status and provide us with the access to your marketplace Seller Account.
3. 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).
If you still have issues on update order status, please contact us.