Sideloading of apps is not enabled on this site
In order to deploy SharePoint hosted app to a site collection using Visual Studio, the site collection must be created using developer site template or have the "Developer" feature enabled on it. We will require to activate this feature using PowerShell as it is a hidden feature. Enable-SPFeature e374875e-06b6-11e0-b0fa-57f5dfd72085 –url https://sharepointdemo.com