grunt postcss “not found”

New to Grunt, trying to get a Gruntfile running for a wordpress project. I’ve got a package.json and Gruntfile.js in my root directory. I want to add grunt-postcss, but when I followed the instructions, I keep getting “Warning: Task “grunt-postcss” not found.” I’ve probably set up Grunt wrong, as no matter what I run, I get task not found. Here’s my Gruntfile contents:

module.exports = function(grunt) {

  grunt.initConfig({
    postcss: {
      options: {
        map: true, // inline sourcemaps
        processors: [
          require('pixrem')(), // add fallbacks for rem units
          require('autoprefixer')({browsers: 'last 2 versions'}), // add vendor prefixes
          require('cssnano')() // minify the result
        ]
      },
      dist: {
        src: 'css/*.css'
      }
    }
  });
  grunt.loadNpmTasks('grunt-postcss');

  grunt.registerTask('default', ['grunt-postcss']);
};