Why is gulp not starting?

Question

gulp.js

var gulp           = require('gulp');
var browserSync    = require('browser-sync');
var sass           = require('gulp-sass');
var autoprefixer   = require('gulp-autoprefixer');
var concatCSS      = require('gulp-concat-css');
var concat         = require('gulp-concat');
var uglify         = require('gulp-uglify');
var cleanCSS       = require('gulp-clean-css');
var imagemin       = require('gulp-tinypng');



// Static Server + watching scss/html files
gulp.task('serve', ['sass', 'scripts', 'bildimg', 'fonts'], function() {

    browserSync.init({
        server: "./bild"
    });
//Следим за изменениями файлов
    gulp.watch("src/sass/*.sass", ['sass']);
    gulp.watch("src/js/*.js", ['scripts']);
    gulp.watch("bild/*.html").on('change', browserSync.reload);
    gulp.watch("bild/**/*.php").on('change', browserSync.reload);
});

// Компилируем sass в css
gulp.task('sass', function() {
    return gulp.src('src/sass/*.sass')
        .pipe(sass().on('error', sass.logError))
        .pipe(autoprefixer({
            browsers: ['last 5 versions'],
            cascade: false
        }))
        .pipe(concatCSS('main.css'))
        .pipe(cleanCSS({level: 2}))
    .pipe(gulp.dest('bild/css/'))
        .pipe(browserSync.stream());
});

// Оптимизируем javascript
gulp.task('scripts', function() {
    return gulp.src('src/js/*.js')
        .pipe(concat('main.js'))
        .pipe(uglify())
        .pipe(gulp.dest('bild/js/'))
        .pipe(browserSync.stream());
});

// Оптимизируем картинки
gulp.task('tinypng', function () {
    gulp.src('src/img/**/*')
        .pipe(imagemin('API_KEY'))
        .pipe(gulp.dest('bild/img/'));
});
// Перемещаем картинки
gulp.task('bildimg', function () {
    gulp.src('src/img/**/*').pipe(gulp.dest('bild/img/'));
});
// Перемещаем шрифты
gulp.task('fonts', function () {
    gulp.src('src/fonts/**/*').pipe(gulp.dest('bild/fonts/'));
});

gulp.task('default', ['serve']);

Answer 1

It also says that the browser-sync module was not found.
$ npm install browser-sync --save-dev