run those commands in your project:

npm install bootstrap
npm install sass
npm install sass-loader

If not exists, create the file

resources/sass/app.scss

and add this:

@import '~bootstrap';

and last add the compilation in the file webpack.mix.js:

mix.sass('resources/sass/app.scss', 'public/css')

After that compile it with npm run dev and you’ll see public/css/app.css

After that you can add this code to your template to import bootstrap:

<link href="{{ asset('css/app.css') }}" rel="stylesheet">