This blog describes how to enable "Document Generation" feature on SharePoint 2013 on-premise server.

Prerequisites: Nintex Live is installed and running on your farm.

1. Go to Central Administration > Nintex Workflow Management > Nintex Live and external Settings
2. Click on "Enable" next to "Enable prerequisite service" on top.
3. Under Nintex Live heading, click on "Activate" next to "Document Generation capability" feature.

The above steps makes the "Document Generation" feature start and running.

Now, you have to add "Document Generation" workflow action to allowed workflow actions on your farm.

1. Go to Nintex Workflow Management > Manage allowed actions
2. Check/Select "Document Generation" workflow action.
3. Click on "OK".

Now, make sure you see "Document Generation" workflow action available for use. For that, create a workflow and from workflow actions, type in "document generation" or go to "Lists and Libraries" category, you see the "Document Generation" workflow action available.