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https://chanakyajayabalan.wordpress.com/2013/01/03/error-occurred-in-deployment-step-install-app-for-sharepoint-sideloading-of-apps-is-not-enabled-on-this-site-sharepoint-2013/
Error occurred in deployment step 'Install app for SharePoint': Sideloading of apps is not enabled on this site. - SharePoint 2013