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.

Ways to debug ASP.NET and Sharepoint webparts

Either run Debug from Visual Studio, which will retract and redeploy the project before deploying and more time consuming

Or, From “Debug”, select “Attach to process…” and attach the debugger to the w3wp.exe processes and start debugging. Make sure to open the browser and browse the site first to start the processes before attaching.

Test emails with SharePoint

use papercut

https://papercut.codeplex.com/

follow the article https://prasadpathak.wordpress.com/2014/10/31/sharepoint-using-papercut-for-testing-the-email-functionality-in-sharepoint/

Make sure the local SMTP server is created and configured and started from IIS 6.0
Disable the Simple Mail Server … service in Services
Set papercut to listen on port 25 local server