This needs to be updated to use an API Key from another site.
Try https://developers.google.com/maps/documentation/javascript/get-api-key
instead of the Dark Sky API.

I started the tutorial "My First PWA" one day before they stopped accepting new Signups, a friend of mine didn't run with the same luck, but, he got something similar called "OpenWeather" and he is using it, you can get more information from here: https://openweathermap.org/api

Can you explain a bit more how to use the API key. The format is not same in both the places

I have a forked version that's working with the Open Weather Map API (https://openweathermap.org/api) - https://github.com/elmorg/your-first-pwapp