Can't get tutorial to work: `Error: The tunnel reported: You provided an invalid key`

I followed the tutorial, but when running
node_modules/.bin/intern-runner config=tests/intern

I get Error: The tunnel reported: You provided an invalid key.

What does that mean? How to fix it?

[jason0x43]

See Step 5. If you're using the example Intern config, it's setup for BrowserStack, so you'll need to provide your BrowserStack username and API key. If you want to run locally, you'll need to change the tunnel to NullTunnel (see https://theintern.github.io/intern/#test-runner).