Concat and Minify not working / Gulp

Question

I've got the following code to concatenate and minify my .css files :

var gulp = require('gulp'),
concatCSS = require('gulp-concat-css'),
minifyCSS = require('gulp-minify-css'),
rename = require('gulp-rename'),
notify = require('gulp-notify'),
browserSync = require('browser-sync').create(),
reload = browserSync.reload,
autoprefixer = require('gulp-autoprefixer');


//Concat CSS
 gulp.task('concat', function () {
 return gulp.src('css/*.css')
  .pipe(concatCSS('css/bundle.css'))
  .pipe(rename('bundle.min.css'))
  .pipe(minifyCSS('bundle.min.css'))
  .pipe(gulp.dest('out/'));
});

 //Watch HTML and reload
   gulp.task('serve', function () {

 // Serve files from the root of this project
    browserSync.init({
    server: {
        baseDir: "./app"
    }
  });

   browserSync.watch("app/*.html").on("change", reload);
});

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

Although Terminal shows no errors the code for concatenation and minifying is not working.

What should I fix to solve the issue? The structure of my project:

UPD!

In my case there were some syntactic mistakes in .css files. Works well after correcting them and removing minifyCSS since it's no longer supported.