Laravel 6.0.3 Not Loading Key Resources App.js or App.css 404 Not Found

The app.css and app.js were missing from a new app. The proper steps had been run as are noted in Jack Robson's answer above. I reran the last two steps:

    php artisan ui vue --auth
    npm install && npm run dev

And it generated the app.css and app.js - all good now.

No idea what had gone wrong in the initial install.

My setup was a little wrong, I mostly forgetting to run npm install and npm run dev command that is required to compile the app.js and app.css files.

Correct setup command sequence was:

laravel new cms
cd cms
php artisan migrate
composer require laravel/ui
npm install
php artisan ui vue --auth
npm install && npm run dev