Error Task 'default' is not in your gulpfile, can anyone help me I can't figure out what the error is?

Question

gulp.js

var gulp        = require('gulp'),
    sass        = require('gulp-sass'),
    browserSync = require('browser-sync'),
    concat      = require('gulp-concat'),
    uglify      = require('gulp-uglifyjs'),
    cssnano     = require('gulp-cssnano'),
    rename      = require('gulp-rename');

gulp.task('sass', function() {
   return gulp.src('app/sass/**/*.sass')
      .pipe(sass())
      .pipe(gulp.dest('app/css'))
      .pipe(browserSync.reload({stream: true}))
});

gulp.task('scripts', function(){
  return gulp.src([
       'app/libs/jquery/dist/jquery.min.js',
       'app/libs/magnific-popup/dist/jquery.magnific-popup.min.js',
    ])
    .pipe(concat('libs.min.js'))
    .pipe(uglify())
    .pipe(gulp.dest('app/js'));
});

gulp.task('css-libs', ['sass'], function(){
  return gulp.src('app/css/libs.css')
  .pipe(cssnano())
  .pipe(rename({suffix: '.min'}))
  .pipe(gulp.dest('app/css'));
});

gulp.task('browser-sync', function(){
  browserSync({
  server: {
  	baseDir: 'app'
    },
    notify: false 
  });
});

gulp.task('watch',['browser-sync', 'css-libs', 'scripts'], function() {
  gulp.watch('app/sass/**/*.sass',['sass']);
  gulp.watch('app/*.html',browserSync.reload);
  gulp.watch('app/js/**/*.js',browserSync.reload);
});