"You must select a product/service or an account for each split line with either an amount or a billable customer"
This is a known bug in the QuickBooks Online Application Program Interface (API) as it is a false error message.
The actual issue is that an expense transaction (Bills, Checks, Credit Card Charges, Credit Card Credits, Purchase Orders, or Vendor Credits) is being imported with a Product or Service mapped to the field LineItem that does not exist in the QuickBooks Online company file. When Transaction Pro Importer attempts to add the Product or Service, Importer cannot create the Product or Service because QB API does not allow developers to set a purchase account on a Product or Service. The workaround is to add the Product or Service to QuickBooks Online company file manually and select a purchase account.