SHELL

command failed with exit code 1: yarn build

what you need to do is just simple: follow these steps:

rm -rf node_modules
yarn cache clean
yarn
yarn start

error command failed with exit code 1. yarn

Delete yarn_lock.json or rm -rf yarn_lock.json (if you are Linux/MacOS user)
Delete node_modules/ or rm -rf node_modules/ (if you are Linux/MacOS user)
Follow the instructions to install the latest Yarn package available from here
Try executing following commands in your project root folder:
yarn install and yarn start

In case if the above approach didn't help:

Try to install latest node.js
Remove node_modules/ and lock file
use npm install and npm run-script