wyliecon I’m using the supplied usb-c to usb-c cable and have even tried a different cable to eliminate that with the same result.
I recommend trying every possible combination of cables and usb ports you can (C to C, A to C in each port). I had the same problem and the only thing that would get the browser to recognize the device was specifically using an A to C cable in a usb 2.0 port. All other configurations failed. I don't know why this happens but I have seen others solve it this way too (which prompted me to try it in the first place).
Also, make sure you have fastboot driver properly installed.