I had this happen yesterday… Was this a split shipment (to save on placement fees)?
If it was, go out of the screen and then back to send to amazon (manage shipments)
You will see all the splits as separate shipments on the list. Just keep choosing a different one of the splits and try and proceed. I guarantee one of them will let you go to the next step.
Had this issue once before and yesterday. This trick worked both times.
Even though these seem like separate shipments, they aren’t. All parts of the split will appear on each individual