As it turns out that Nessus Essentials is having trouble sending out e-mails. Ran into it after installing Nessus on a Kali VM. Filled out the form and although Nessus stated, that e-mail sent successfully, no message arrived. Not even after a few more attempts. Fortunately there is a quick work around, I wish I just had turned to Tenable’s website a bit sooner. To activate Nessus Esstentials just use Tenable’s own website to request the activation code. Just go fill out the form at https://www.tenable.com/products/nessus/activation-code and don’t wait for the one from your own installer, as it probably will never arrive.. Happy Scanning!