Attempting to use fastboot to install TWRP; Windows cmd doesn't see phone

[AC Question]

I have downloaded and installed Fastboot and ADB. I have saved the correct version of TWRP to a folder within that directory.

When trying to run the command "adb reboot bootloader" windows is returning the following:

error: device '(null)' not found

Windows (7, 64-bit) sees my phone. I also have the phone connected as a media device, once I plugged it into my computer. USB Debugging has been enabled through the Developer Settings I unlocked before plugging my phone in.

As far as I can tell, I did everything right so far. What did I miss?

 

[srkmagnus]

Make sure you have the appropriate drivers installed. I would also suggests reinstalling adb and see if that fixes the problem.