There is a problem with the logic in CreateOrUpadeteShipping in
https://github.com/nopSolutions/nopCommerce/blob/develop/src/Plugins/Nop.Plugin.Shipping.ShipStation/Services/ShipStationService.cs
It is trying to write a shipmentItem before writing the shipment to get and use the Id
See ikssue and quick fix here
https://www.nopcommerce.com/en/boards/topic/85274/430-how-to-get-shipments-in-order-from-plugin