I cant run this in dev mode I get Cannot find module 'node-sass'

I´m new to React and following instructions here, 

> npm install
> npm run serve
> npm run dev

Then in browser windows I get error:

> Cannot find module 'node-sass'

I posted an SO question about this:
https://stackoverflow.com/questions/56826025/how-to-fix-this-error-when-trying-to-start-microsoft-vscode-react-sample?noredirect=1#comment100204375_56826025