I'm not all that great at using adb and what not, but I was having the same issue when trying to sideload 6.0 on my nexus 5. I was blaming it on windows 10 not working with it, since I updated the drivers manually, followed different tutorials and got nowhere when typing adb devices.
However I think I tried "fastboot devices" and it popped up right away and I was then able to sideload. Idk if that helps or is the same thing?
Are you also on windows 10?
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