If you are getting this error ” Error occurred in deployment step ‘Install SharePoint Add-in’: Sideloading of apps is not enabled on this site ” when debugging a SharePoint Add-in it means site you are using is a Production site. Sideloading of Apps is not enabled on any site by default as specially Production sites since it allow developers to load Apps which are still under development and not published.
If you are going to do some tests then enable it using PowerShell for SharePoint Online.
PowerShell code can be downloaded from MSDN code repository from Here
To disable Sideloading you can download code from here