Getting error when running the grunt task

[xRahul]

I am not proficient in grunt, but after following some things online, I built Gruntfile. But when running the task, I'm getting an error:

module.exports = function(grunt) {

  // Project configuration.
  grunt.initConfig({
    pkg: grunt.file.readJSON('package.json'),
    uncss: {
      index: {
        files: [{
          expand: true,
          cwd: "_site/",
          src: "**/*.html",
          dest: "_site/"
        }]
      }
    }
  });

  grunt.loadNpmTasks('grunt-uncss-inline');
  // Default task(s).
  grunt.registerTask('default', ['uncss_inline']);
};

Registering Gruntfile tasks.
Reading package.json...OK
Parsing package.json...OK
Initializing config...OK

Registering "grunt-uncss-inline" local Npm module tasks.
Reading /Users/slx/Documents/GitHub/xRahul.github.io/node_modules/grunt-uncss-inline/package.json...OK
Parsing /Users/slx/Documents/GitHub/xRahul.github.io/node_modules/grunt-uncss-inline/package.json...OK
Loading "uncss-inline.js" tasks...OK
+ uncss_inline
Loading "Gruntfile.js" tasks...OK
>> No tasks were registered or unregistered.

Running tasks: uncss_inline

Running "uncss_inline" task
[D] Task source: /Users/slx/Documents/GitHub/xRahul.github.io/node_modules/grunt-uncss-inline/tasks/uncss-inline.js
>> No "uncss_inline" targets found.
Warning: Task "uncss_inline" failed. Use --force to continue.
Error: Task "uncss_inline" failed.
  at Task.<anonymous> (/Users/slx/Documents/GitHub/xRahul.github.io/node_modules/grunt/lib/util/task.js:198:15)
  at Task.runTaskFn (/Users/slx/Documents/GitHub/xRahul.github.io/node_modules/grunt/lib/util/task.js:247:9)
  at Task.<anonymous> (/Users/slx/Documents/GitHub/xRahul.github.io/node_modules/grunt/lib/util/task.js:293:12)
  at Task.start (/Users/slx/Documents/GitHub/xRahul.github.io/node_modules/grunt/lib/util/task.js:302:5)
  at Object.grunt.tasks (/Users/slx/Documents/GitHub/xRahul.github.io/node_modules/grunt/lib/grunt.js:155:8)
  at Object.module.exports [as cli] (/Users/slx/Documents/GitHub/xRahul.github.io/node_modules/grunt/lib/grunt/cli.js:27:9)
  at Object.<anonymous> (/Users/slx/.config/yarn/global/node_modules/grunt-cli/bin/grunt:44:20)
  at Module._compile (module.js:652:30)
  at Object.Module._extensions..js (module.js:663:10)
  at Module.load (module.js:565:32)
  at tryModuleLoad (module.js:505:12)
  at Function.Module._load (module.js:497:3)
  at Module.require (module.js:596:17)
  at require (internal/module.js:11:18)
  at Object.<anonymous> (/Users/slx/.config/yarn/global/node_modules/grunt/bin/grunt:3:1)
  at Module._compile (module.js:652:30)
  at Object.Module._extensions..js (module.js:663:10)
  at Module.load (module.js:565:32)
  at tryModuleLoad (module.js:505:12)
  at Function.Module._load (module.js:497:3)
  at Function.Module.runMain (module.js:693:10)
  at startup (bootstrap_node.js:188:16)
  at bootstrap_node.js:609:3


Aborted due to warnings.

If anyone knows what I did wrong, let me know.

[sparanoid]

>> No "uncss_inline" targets found.

It seems you didn't register the task correctly.