Topic
The Declare as Record action appears in the tool box when you create a workflow on a Library but the action is not there when you create a workflow on a list.
Instructions
To enable this action on lists for SharePoint 2013 navigate to Central Administration > Nintex Workflow Management > Manage allowed actions.
For SharePoint 2016, 2019 navigate to Central Administration > Nintex Administration > Action Settings.
Click on Declare as Record link and scroll down to Document Libraries Only.
Set this option to "All lists and Libraries" and the action will now appear on lists.