I think the best way to find the concrete error with the emulator is to start it within a terminal.
And what’s the cause of that? As you can see, there could be many causes. It seems that “Waiting for target device to come online …” is a generic message that appears, always, when the emulator can not start properly. For this you could copy and paste your command line from “Run” or “AVD” Android Studio console. You need to discover the reason of this… You could run your emulator from the command line. I had a similar problem… Android Emulator doesn’t open.
