This error typically occurs when your QuickBooks Desktop application is configured to disallow negative quantity adjustments.
Log in to QuickBooks as an admin and ensure that you are in single-user mode.
Click Edit > Preferences > Item and Inventory > Company Preferences > Warn if not enough inventory to sell > disable the option “Don’t Allow Negative quantities” and select other available options as per your requirement > OK.
Once done then try to post the order again from Webgility Software.
Feel free to reach out to Support by initiating a chat if the issue persists.