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 API as it is a false error message. What is really happening is that you are importing an expense transaction (eg bill, check, credit card charge/credit, purchase order or vendor credit) that you have a product or service mapped to the QuickBooks field, LineItem, that does not exist in your QuickBooks Online company file. When we try to add the product because the QuickBooks API does not allow developers to set a purchase account on a product or service we cannot create the item. The workaround is to add this product or service to your QBO products or services list and select a Purchase Account.