Also, what device are you targeting? A common gotcha according to this walk through guide Getting Started with Elixir and Nerves: Part 1 is to make sure you set the right MIX_TARGET environment variable for the device you are targeting.
Many commands won’t run if you don’t set MIX_TARGET and its not always obvious what is wrong. This was mentioned to me at the conference but it has bitten me several times since.
It may be useful to know what’s your target device and how are you trying to connect it, because there are many possibilities: USB data cable, Ethernet, WiFi… But these options also depends of your target.
If you give us more information about it, I am sure someone could help you to start with Nerves!