When invoices are synced to QuickBooks, each invoice line is linked to a product/service set up in your QuickBooks account, which must be linked to an income account (for example, you may have an item set up for Halaxy merchant fees). You must link the relevant product/service in QuickBooks to an income account.
Follow the steps below:
In QuickBooks, click Sales > Products and services.
Search for and click the product/service that needs to be linked (e.g. "HX - Merchant fees").
Tick the checkbox labelled "I sell this product/service to my customers" and save.
RESULT: Your invoices should now sync successfully.