So the problem is here that you are telling Netlify to run the command “yarn build” but:
"scripts": {
"start": "react-scripts start",
"build": "react-scripts build",
"test": "react-scripts test --env=jsdom",
"eject": "react-scripts eject",
"predeploy": "npm run build",
"deploy": "gh-pages -d build"
},
yarn is only available in our build environment when a yarn.lock file exists in the root of your repo (or in the base directory, if set), OR if you add it in package.json and THAT file exists in the root of your repo (or in the base directory, if set)
You might also find this generic debugging information useful.
This Support Guide contains a ton of useful debugging tips.