Although I am still new to Waze, I have been able to resolve a few of my issues with Voice Commands and thought I would post that information here in case it helps someone else. I use a Samsung Galaxy Core Prime phone, Android 5.1.1 and Waze Version 18.104.22.168
I started with no access to Voice Commands at all. Went into Waze > Settings > Voice Commands and Enabled and set to Activate with 3 fingers. Unfortunately, my Samsung Galaxy Core Prime does not recognize 3 fingers as a "gesture". When that didn't work, I changed the selection to 3 fingers or wave.
With that I was able to get the Voice Command screen with the microphone. But, it would always show as "Waiting". It seemed to not hear me. I got out of Waze and went into my phone settings. Under System I chose Language and input > Voice input. Apparently my phone did not default to any type of voice input. I first chose Enhanced Google Service, but for some reason (which I don't understand yet) that didn't work. Then I chose Basic Google Recognition and exited settings.
I went back into Waze with that setting, and now Voice Commands work. It doesn't always recognize what I am saying, but I have able to do several reports by voice. If I figure that out, I'll let you know.
Hopefully, this is helpful to someone.