Due to Amazon policy, you are not allowed to delete any shipments in a shipping plan if you do not delete them all. For more information on this policy, see the below:
If you need to delete a shipment, you'll have to delete any working shipments already created as well:
If you get to the Proposed Shipping page and don't like the splits, you can choose to Decline Shipments and Go Back and try again later.
If you notice you have duplicate shipments, before you create any, you'll need to close and restore the batch to correct the issue.