If you're unable to send invoices in QuickBooks Desktop, several issues might be causing the problem. When you cannot send invoice in QuickBooks Desktop, common solutions include ensuring your email settings are correctly configured, checking your internet connection, and verifying that QuickBooks is updated to the latest version. Additionally, your firewall or antivirus settings might be blocking the connection, or there may be issues with your email provider. Reviewing and adjusting these settings can often resolve the issue, allowing you to send invoices without trouble.