gulp autoprefixer does not create file

Question

i am quite new to autoprefixer, i did just like the documentation but when i type "gulp styles" into the terminal it did not create any file in the destination folder

here is my gulpfile.js code

const gulp = require('gulp');
const autoprefixer = require('gulp-autoprefixer');

gulp.task('styles', () =>
gulp.src('css/styles.css')
  .pipe(autoprefixer({
    browsers: ['last 2 versions'],
    cascade: false
  }))
  .pipe(gulp.dest('build'))
);
gulp.task('watch', function () {
  gulp.watch('css/styles.css', ['styles']);
});

and here is my css code

.container {
  display:flex;
  flex-direction:column;
  align-content:center;
  justify-content:center;
}

.box1 {
  flex:10 5 450px;
}

.box2 {
  flex:1 1 360px;
}