You get an error that reads, "ERROR OCCURRED: A business validation error has occurred while processing your request: Business Validation Error: Select a product or service" during the sync.
Here's what the error looks like in the Sync Log:
At first glance, the error doesn't make much sense, because each of the invoices has items on it.
But what the error message really means is that you do not have an Income Account defined in QuickBooks for one or more items on the invoice.
Here a screenshot of Invoice I190624368:
Clearly, it as an item on it.
But what we can't see is that this item in QuickBooks Online does not have an income account associated with it.
To fix this error, edit the item in QuickBooks Online, select an Income Account and Save.
Then re-run the sync.