You might find QuickBooks unable to send invoices if the email settings are incorrect. Therefore, check and make sure that the email is set up correctly, and the email address and server information are correct. Moreover, check the Windows Firewall and antivirus, and ensure they don’t interfere with QuickBooks sending invoices. 

Read more about this issue:- https://proadvisorsolutions.com/blog/quickbooks-not-sending-invoices/