The warning message you are getting is a result of a validation done by MYOB. You probably have inventory tracking enabled for your items, and when you are trying to export a sale for that item, there is not enough quantity on hand in MYOB yet.

Removing inventory tracking could be an option. Otherwise what you can do is make sure you export purchases first so that your stock goes up before exporting invoices. Do you use our software to export purchase orders? If you export POs at the same time as invoices, then our software takes care of exporting POs first and then invoices.