If you are a SharePoint Add-in or App developer, you could have probably come through this issue atleast once.
“Sideloading of apps is not enabled on this site.”
Sideloading is a technique that most of the app vendors allow in order to test or debug the apps. In other words, it means deploying an app with a development tool rather than going through the correct secure procedure. Sideloading is available with all the smartphone platforms including Android.