When I faced this problem I was on Android Studio 3.1 version. I tried a lot of approach above, nothing worked for me ( Don't know why :/ ). Finally I tried something different by my own. My approach was:
Before going to bellow steps make sure
*Your "Google USB Driver" package is installed ("Tools" -> "SDK Manager" -> Check "Google USB Driver" -> "Apply" -> "Ok").
*If you are trying to access with emulator then check "Intel x86 Emulator Accelarator(HAXM installer)" is instaled. ("Tools" -> "SDK Manager" -> Check "Intel x86 Emulator Accelarator(HAXM installer)"" -> "Apply" -> "Ok")
Hope this will help somebody like me.