Define which packages will be use firstly.

gulpfile.js

var gulp           = require('gulp'),
    concat         = require('gulp-concat'),
    uglify         = require('gulp-uglify'),
    rename         = require('gulp-rename'),
    sass           = require('gulp-sass'),
    sourcemaps     = require('gulp-sourcemaps'),
    livereload = require('gulp-livereload');