Allow workflows to change approval status

Follow the instructions of the last answer at the end of the page.

https://social.technet.microsoft.com/Forums/office/en-US/9292c3ef-b456-4115-8b9c-bfa63782dfd5/workflow-can-use-app-permissions-feature-is-not-activating?forum=sharepointadmin

If Workflow Service Application is not displayed, enabled it by launching the Farm Configuration Wizard.