​Thanks for trying Sam :)

Well, I just cloned the project from GitHub and had no issues. Here are the steps of what I did:

git clone https://github.com/cutemachine/hyperbulma.git
cd hyperbulma
rm -rf .git
yarn
yarn start

I'm using the same version of yarn (1.3.2). When you call yarn start it looks into the package.json and tries to find a script called start and executes it. So what gets run is parcel src/index.html.

I would guess that it is an issue with Parcel or Babel. Which OS are you on?

Have you tried running npm run start?