mapserver-wasm needs to be build before you can run the demo. Alternatively you can swap out the file:../ string inside the package.json by a valid published version of mapserver-wasm.
The demo does not come with necessary GeoPackage data files. Please serve your own test files under the public folder and reference them in config.js.
Now you can start the demo on Port 8080 with the following command:
npm ci && npm start