QuickBooks Unable to Send Invoices Error appears on the screen when users are using incorrect email preference or outdated version of the application while sending invoices via Gmail or Outlook. This error occurs due to incorrect settings of email preferences. If you want to get rid of QuickBooks Unable to Send Invoices Error, then you must run QuickBooks Diagnostic Tool and make sure to update the application with the latest version. To know about the complete process, follow the steps mentioned in the blog or else contact us at (844)-932-1139.