Try this. Make sure developer options is enabled and also USB debugging as Josh already commented and scroll down to "default USB configuration" (attached screenshot showd it) and open that setting and while your phone's plugged in to your PC change the setting there from its default setting "no data transfer" to "file transfer" but keep a close eye on your device because a box will pop up and it will only be present for a few seconds. Checkmark that box. By doing this you are allowing your phone and your computer to communicate. You should be good then just make sure you are plugged in properly and you are using a decent USB cable. I use an older Toshiba laptop that's ancient and I've used it many years now and there has been more than one or two times where I joggled my USB cord , very slightly where it plugs in to my laptop and that was always enough to get it a good enough connection to do what I need it to do. Hope this helps.I flashed today and I can't use anything adb root related, even though the build appears to be a userdebug build.