Netmirror is not on the Play Store, so you must "sideload" it. Select your device below for easy instructions.
You likely have an older version of Netmirror (or a differnet fork) installed. Uninstall it completely first. Also check if you have at least 100MB free space.
This means the download was corrupted. Delete the file and download it again. Also, ensure your device is running Android 5.0 or higher.
Google Play Protect may flag apps not on the Play Store. Click "More Details" -> "Install Anyway". The app is safe.