Approval Workflow Missing! on Sharepoint 2013
One or the other day, we end up with the following scenario.
“The Approval Workflow is missing!”
I opened the list & list settings, tried to add a workflow & I cannot see the Approval workflow.

To use legacy Sharepoint 2010 Workflow installed by Default (WWF 3.0)
Feature that needs to be activated is as shown below.


The out of the box workflows that ship with SharePoint 2013 are still SharePoint 2010-based workflows. Since SharePoint 2013 uses the Workflow Manager service, you must use SharePoint Designer to create 2013-based workflows. The out of the box approval workflow for 2013 is the same one that was used in 2010.
The only platform available when you first install SharePoint Server 2013 is the SharePoint 2010 Workflow platform. The SharePoint 2013 Workflow platform and the Project Server platform require additional steps. The three workflow platforms are outlined in the following table.


To install Sharepoint 2013 Workflows (WWF 4.0):

You need to configure workflow for SharePoint 2013 as well Workflow manager. Follow the steps given in below Microsoft article to make it up and running.