unable to get examples to work

Followed the instructions for the babylon demo folder. it says go to root folder to run `npm install` and then `npm run start-babylon`. Two browser tabs open up, but when clicking on any of the demo links get lots of compile error like this:

ERROR in ./packages/amazon-sumerian-hosts-babylon/src/Babylon.js/HostObject.js 8:0-64

Module not found: Error: Can't resolve '@babylonjs/core/Loading/sceneLoader' in '/Users/username/pp/amazon-sumerian-hosts/packages/amazon-sumerian-hosts-babylon/src/Babylon.js'

Are there some missing instructions?