npm start - npm ERR! missing script: start

You are missing the entry start in the scripts section of your package.json. If you want to compile the file using npm start then, you should copy node-sass sass/main.scss css/style.css -w and paste it as a value for start. You can add any command you would like to execute as the value for start.

Having said that, if you do npm "compile:sass" it will compile your sass files if you don't want to change your package.json.

"scripts": {
    ...
    "compile:sass": "node-sass sass/main.scss  css/style.css -w",
     "start": "node-sass sass/main.scss  css/style.css -w"
  },

Because there is no start script... Just change compile script to start script... Then it will compile your sass files...

"scripts": {
  "start": "node-sass sass/main.scss  css/style.css -w"
},