Plugin is not a function Grunt.js

JavaScript
1

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I get this error:

Running "postcss:dist" (postcss) task
Fatal error: plugin is not a function

I think it’s coming from this grunt.js file when I execute: grunt build

postcss: {
            options: {
                processors: [
                    require('pixrem')(), // add fallbacks for rem units
                    require('autoprefixer')(), // add vendor prefixes
                    require('cssnano')() // minify the result
                ]
            },
            dist: {
                src: 'dist/css/main.css'
            }
        },

No clue as how to fix it because I bought the template and didn’t look at the installation. So didn’t know it should be compiled with grunt.js. My bad I know.

I fixed a couple of deprecated stuff. But now I’m at the end of my knowledge. Pretty sure I bought an outdated template, which is useless now.