When posting Order to QuickBooks Online you may encounter this error message
You must select a product/service or an account for each split line with either an amount or a billable $$customer$$
The Shipping account may not be set in QuickBooks Online.
Login to QuickBooks Online, go to Settings > Company Settings.
Click on Sales > Sales Form Content > Make sure the Shipping Is set to On.
Now click on the Advanced Tab and make sure the Shipping has an account linked to it.