Grunt "watch.sass.files" missing

Question

Yes, what you've done is put the entire Sass configuration inside the watch config. Grunt does not work this way. Instead, change your config to something like this:

module.exports = function(grunt) {

    // 1. All configuration goes here
    grunt.initConfig({

        pkg: grunt.file.readJSON('package.json'),

        // 2. Configuration for concatinating files goes here.

        // Concatonate various files into one
        concat: {
            dist: {
                src: [
                    'js/vendor/*.js', // All JS in the libs folder
                    'js/plugin.js', // All JS in the libs folder
                    'js/global.js'  // This specific file
                ],
                dest: 'js/build/production.js',
            }
        },

        // Creates a minified version of the javascript files of the project
        uglify: {
            build: {
                src: ['js/vendor/*.js', 'js/plugin.js', 'js/global.js'],
                dest: 'js/build/production.min.js'
            }
        },
        // Compile sass to css
        sass: {
            dist: {
                options: {
                    style: 'compressed'
                },
                files: {
                    'css/build/style.css': 'sass/style.scss'
                }
            }
        },

         // Minifies automatically the images of the project
        // imagemin: {
        //     dynamic: {
        //         files: [{
        //             expand: true,
        //             cwd: 'images/',
        //             src: ['**/*.{png,jpg,gif}'],
        //             dest: 'images/build/'
        //         }]
        //     }
        // },

        // Watches for changes done on the project and builds the commands if neccesary
        watch: {

             options: {
                livereload: true,
            },

            scripts: {
                files: ['js/*.js'],
                tasks: ['concat', 'uglify'],
                options: {
                    spawn: false,
                },
            },

            css: {
                files: ['css/*.scss'],
                tasks: ['sass'],
                options: {
                    spawn: false,
                }
            }
        }

    });

    // 3. Where we tell Grunt we plan to use this plug-in.
    grunt.loadNpmTasks('grunt-contrib-concat');
    grunt.loadNpmTasks('grunt-contrib-uglify');
    // grunt.loadNpmTasks('grunt-contrib-imagemin');
    grunt.loadNpmTasks('grunt-contrib-sass');
    grunt.loadNpmTasks('grunt-contrib-compass');
    grunt.loadNpmTasks('grunt-contrib-watch');

    // 4. Where we tell Grunt what to do when we type "grunt" into the terminal.
    grunt.registerTask('default', ['concat', 'uglify', 'sass', 'compass', 'watch']);

};