Help - Run error - mix.postCss(...).purgeCss is not a function

[jLynx]

Hi Team

I am struggling to get this working in Laravel. This is what I have done so far:
npm install postcss-purgecss-laravel --save-dev
npm install tailwindcss --save-dev

mix.postCss('public/css/styles2.css', 'public/css/test', [
        require('tailwindcss')(),
        require('postcss-purgecss-laravel')({
            safelist: [/hljs/],
        }),
    ])
    .purgeCss();

Then I run npm run production

But all I get is the error:
TypeError: mix.postCss(...).purgeCss is not a function

Is there something else I need to install, or have I just done it incorrectly?