yarn start run is not a command provided by Sage, so I’m not sure what you expect it to do. You can see the commands Sage provides and what they do in package.json:
"scripts": {
"build": "webpack --progress --config resources/assets/build/webpack.config.js",
"build:production": "webpack --env.production --progress --config resources/assets/build/webpack.config.js",
"build:profile": "webpack --progress --profile --json --config resources/assets/build/webpack.config.js",
"start": "webpack --hide-modules --watch --config resources/assets/build/webpack.config.js",
"rmdist": "rimraf dist",
"lint": "npm run -s lint:scripts && npm run -s lint:styles",
"lint:scripts": "eslint resources/assets/scripts resources/assets/build",
"lint:styles": "stylelint \"resources/assets/styles/**/*.{css,sass,scss,sss,less}\"",
"test": "npm run -s lint"
},