Why is the same Youtube API request returning 401 in a different project?

I am fetching playlistItems from the Youtube Data API via this URL:

URL = `https://youtube.googleapis.com/youtube/v3/playlistItems?part=snippet%2C%20contentDetails&playlistId=${playlistId}&maxResults=${50}&pageToken=${nextPageToken}&key=${API_KEY}`

it works fine in a project I'm using for experimentation.

Now it was time to copy 🍝 the code into my actual application.

The only thing I changed was that I put the API_Key inside my .env and set it via:

const API_KEY = process.env.YT_API_KEY

I still don't understand why, but the URL ends up having key=undefined at the end:

I am setting it inside .env like this:

Now to the real reason why I'm seeking help:To fix the issue, I set the API_KEY directly inside the file that makes the fetch.

But I get the same 401 error as above (except key= is not undefined anymore)

And when I click the link in the error message, I can look at the JSON object, that I seek. So the URL is definitely working. As it (still) does in my other project.

So what leads to the 401 error then?

I had the idea, that maybe I need to change something in the cloud platform credentials settings. But my API key doesn't have any restrictions.