If you are getting the message "{"error":"Could not find a Listing with listing_id = 12345678"}" this can be due to missing data from an unshipped order where the Etsy listing has been deleted or removed.
You will need to manually process the order with the missing listing in XPS Ship and update the tracking number in Etsy.
The error should disappear after a few minutes and you can continue shipping your Etsy orders.