Nintex Live ID is an external end Point and you should copy it from the SharePoint 2013 as a preparation to a SharePoint 2016 migration. You can save it temporarily on a notepad. After creating a new SharePoint 2016 farm you will need paste the copied Live ID to the 2016 SharePoint farm Live ID settings.
Instructions:
-
On the Central Administration Home page, select Nintex Live Management.
-
Click Connection settings.
-
Copy the existing value of the ID to a file for later use.
Later in the upgrade process, this ID will be restored in the SharePoint 2016 environment. See Restore the external connection end point ID.
-
Click OK.
for more information on migrating Nintex workflows to SharePoint 2016 refer to this post :
Migrating Nintex workflows and history from 2010 to 2016