The ERROR: Something went wrong. Please try again later. comes directly from Amazon in Step2: Pack or Step 4: Confirm Transportation.
The error typically occurs when a shipping plan enters a state where further actions are disallowed. As a result, any subsequent changes to the shipment may not succeed within the intended API architecture, causing the error to persist even after attempting necessary corrections.", but also said, "This usually occurs when the first request is still executing at the time the second one was sent."
To resolve:
1. Download the Pack List(s) from Pack Step if the items are already packed.
2. Cancel the Shipment Plan.
3. Create the Shipment Plan again.
4. (optional) Remove any errored items from the batch and create shipment plan without the errored items.
5. Upload the Pack List(s) in Step 2.
6. Progress the batch.