Help rooting? Sprint N6

I'm trying to root my Nexus 6 on Sprint following the [HOW-TO] The Nexus 6 All-In-One Beginner's Guide by xBeerdroiDx. I'm on step 3. Unlocking the bootloader, the problem i'm having is my phone not being recognized when I open the command prompt and type in "Fastboot devices." I have the Dev options turned on, oem unlocking is enabled and so is USB debugging. Any ideas as to why this might be happening? I've followed the instructions to a T, but there's no support for what to do when this happens...

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