Gulp - why does compilation, browser sync and the rest not work?

Question

Node.js

I downloaded the ready-made gulp collector from git, ran it, then tried to change scss/sass , and nothing changed in the main.min.css file and on the site as well.
gulpfile.js

var syntax         = 'sass', // Syntax: sass or scss;
    gulpVersion    = '4'; // Gulp version: 3 or 4
    gmWatch        = false; // ON/OFF GraphicsMagick watching "img/_src" folder (true/false). Linux install gm: sudo apt update; sudo apt install graphicsmagick

var gulp          = require('gulp')
    sass          = require('gulp-sass'),
    browserSync   = require('browser-sync'),
    concat        = require('gulp-concat'),
    uglify        = require('gulp-uglify'),
    cleancss      = require('gulp-clean-css'),
    rename        = require('gulp-rename'),
    autoprefixer  = require('gulp-autoprefixer'),
    notify        = require('gulp-notify'),
    rsync         = require('gulp-rsync'),
    imageResize   = require('gulp-image-resize'),
    imagemin      = require('gulp-imagemin'),
    del           = require('del');

// Local Server
gulp.task('browser-sync', function() {
  browserSync({
    server: {
      baseDir: 'app'
    },
    notify: false,
    // open: false,
    // online: false, // Work Offline Without Internet Connection
    // tunnel: true, tunnel: "projectname", // Demonstration page: http://projectname.localtunnel.me
  })
});

// Sass|Scss Styles
gulp.task('styles', function() {
  return gulp.src('app/'+syntax+'/**/*.'+syntax+'')
  .pipe(sass({ outputStyle: 'expanded' }).on("error", notify.onError()))
  .pipe(rename({ suffix: '.min', prefix : '' }))
  .pipe(autoprefixer(['last 15 versions']))
  .pipe(cleancss( {level: { 1: { specialComments: 0 } } })) // Opt., comment out when debugging
  .pipe(gulp.dest('app/css'))
  .pipe(browserSync.stream())
});