I'm sure you've probably found this link, but it does look like a known issue with Magento 2.3.0 and they're working on it. If a temporary workaround is to be had it'll be revealed here:
Update: I was able to make this error go away and allow shipping orders by turning off Magento managing stock.
Stores > Configuration > Catalog > Inventory > Product Stock Options > Manage Stock
Set this value to "No" and you can bypass the error.
This does prevent Magento from managing your stock, so if your flow relies on Magento reducing your stock on hand when an order ships, don't change this setting. In our case we do not need Magento to worry about stock values so the fix worked for us. If you do need this feature, you'll need to follow the issue report at the link above and watch for either an update to Magento or a shared hotfix.