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Make sure that you have the latest version of iTunes
Maybe you get the unknown error 0xe8000015 message because the iTunes version installed in your computer is outdated. In this case, all you need to do is to update it, either via iTunes itself or by downloading it from Apple's website. Once you have updated iTunes, try again.
To check if your iTunes version is up-to-date (and update it, if it's not), open iTunes, click on Help and select Check for Updates.
Maybe your computer's antivirus software doesn't "trust" your iOS device, resulting in "unknown error 0xe8000015." Open your antivirus software and check if there is any sign that your device is being blocked. If it is, all you need to do is to tell the antivirus that your device is not a "threat." If you are not sure how to access the antivirus and check its settings, feel free to consult the developers, on their website.
Plug in your device in another USB port of the computer
Maybe the USB port your device is currently plugged in is broken. Unplug it from that USB port and plug it in another port. Your device and the computer should react (with a sound and the display should turn on) once a successful connection is established.
Maybe the USB (lightning) cable is the causer of the unknown error 0xe8000015 error message. Borrow another USB cable and connect your device to your computer. If you don't see the error message and your device functions as expected, replace the cable.
Maybe the device's or computer's USB port is dirty (e.g. clogged with dust lumps) and prevents a successful connection. Sip some isopropyl alcohol on a piece of cloth and clean the ports, gently, to avoid damaging them. Try connecting the devices again after 5 minutes.