"Unknown SKU" during the Shipping Plan Phase

Here is the most common error message when viewing your shipping plan. "Unknown SKU.  The SKU for this product is unknown or cannot be found".

This means that the plan was created before your items had a chance to upload to your seller account.  To fix this issue in the future, please wait at least 5 minutes before creating a shipping plan.  You can also check when the upload is complete by going to your Amazon Seller Central account.  Go to Inventory - Upload Products via Upload - Monitor Upload Status.  Look for your last batch and wait for it to say: Done.

To fix this issue now, create another shipping plan and select the most recent batch.  Use another plan name.