When clicking on the Site Actions menu, the option for Nintex Workflow is missing.

Activate the Nintex Workflow Web Application Feature. To do so, navigate to:

• Central Administration
• Nintex Workflow Management (SharePoint 2013) or Nintex Administration (SharePoint 2016+)
• Web Application Activation
• Select your Web Application (or check the box for 'All content web applications)
• Click 'Activate'.

This behavior occurs when the following two entries are not added to the web.config file for your Web Application:

<SafeControl Assembly="Nintex.Workflow.ServerControls, Version=1.0.0.0, Culture=neutral, PublicKeyToken=913f6bae0ca5ae12" Namespace="Nintex.Workflow.ServerControls" TypeName="*" Safe="True" />
<SafeControl Assembly="Nintex.Workflow.ServerControls, Version=1.0.0.0, Culture=neutral, PublicKeyToken=913f6bae0ca5ae12" Namespace="Nintex.Workflow.ServerControls.WebParts" TypeName="*" Safe="True" />