Enabling ‘Unknown Sources’ on Android Oreo or Higher
Starting with Android 8.0, the process of sideloading apps has changed dramatically. No longer is there a system-wide “Unknown Sources” setting — instead, it’s now handled on a per-app basis.
This means that when you download an APK with your browser, for instance, you’ll have to give your browser permission to install apps. Same goes if you try to install an APK from your favorite file explorer — the file explorer will need explicit permission to install apps as well.
While this is a pretty big departure from the way things worked before, it’s actually pretty simple. Just download APKs as you normally would, but when you launch the APK, you’ll be prompted to allow the permission. Tap “Settings” on the prompt, then enable the switch next to “Allow from this source” on the following screen. From there, hit your back button, then you can resume installation.