How do you want to use mqtt.js?
This error occurs only if you download mqtt.js sources and install all dependencies.
But you don't need to download mqttjs sources for developing your own applications that use mqttjs.
You should init new node project and simply install mqttjs as a dependency.
npm init
npm i mqtt
Alternatively if you want to continue developing mqttjs library you should clone mqttjs repo
git clone https://github.com/mqttjs/MQTT.js.git
then you should update zuul-ngrok dependency
npm i git+https://git@github.com/rase-/zuul-ngrok.git
and then you can install all dependencies of mqttjs
npm i
I hope this helps you ;)