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.
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.
for more information on migrating Nintex workflows to SharePoint 2016 refer to this post :