I'm here to help make sure you're able to sync seamlessly with QuickBooks.
Just to clarify, are you getting the error when trying to sync your transactions from a supported third-party application? If so, may I ask what specific app it is? A screenshot of the actual error would be very helpful so we can further check on its root cause.
In the meantime, you'd want to make sure that a line item is added to the transaction, then save the transaction.