TypeError: dest.on is not a function?

Question

css

I decided to add some plugins for gulp, but after installing them and making changes to the gulpfile, I started getting the TypeError: dest.on is not a function error . With what it can be connected?
throws this error only when running cssnano and sass

gulp.task('cssnano', function() {
    return gulp.src('app/css/*.css')
      .pipe(cssnano())
      .pipe(gulp.dest('dist/css'));
});

gulp.task('sass', function(){ 
    return gulp.src('app/sass/**/*.scss') 
        .pipe(sass()) 
        .pipe(autoprefixer(['last 15 versions', '> 1%', 'ie 8', 'ie 7'], { cascade: true })) 
        .pipe(gulp.dest('app/css'))
        .pipe(browserSync.reload({stream: true}))
});

everything else works fine
Screenshot of the error
And yet, how can I configure gulpfile more correctly?
gulpfile itself

let browserSync   = require('browser-sync'),
    gulp          = require('gulp'),
    concat        = require('gulp-concat'),
    uglify        = require('gulp-uglifyjs'),
    cache         = require('gulp-cache'),
    del           = require('del'),

    postcss       = require('gulp-postcss'),
    autoprefixer  = require('autoprefixer'),
    cssnano       = require('cssnano'),
    fonts         = require('postcss-font-magician'),

    sass          = require('gulp-sass'),
    media         = require('gulp-group-css-media-queries'),
    notify        = require('gulp-notify'),
    pug           = require('gulp-pug'),
    sourcemaps    = require('gulp-sourcemaps');


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

gulp.task('sass', function(){ 
    return gulp.src('app/sass/**/*.scss') 
        .pipe(sass()) 
        .pipe(autoprefixer(['last 15 versions', '> 1%', 'ie 8', 'ie 7'], { cascade: true })) 
        .pipe(gulp.dest('app/css')) 
        .pipe(browserSync.reload({stream: true})) 
});

gulp.task('map', function() {
  gulp.src('app/css/*.css')
    .pipe(sourcemaps.init())
    .pipe(sourcemaps.write())
    .pipe(gulp.dest('dist'));
});

gulp.task('postcss', function () {
  const processor = ([
    autoprefixer({browsers: ['last 15 versions', '> 1%', 'ie 8', 'ie 7']}),
    //cssnano(),
    fonts()
  ]);
  return gulp.src('./app/sass/*.sass')
    .pipe(sass().on('error', notify.onError()))
    .pipe(postcss(processor))
    .pipe(media())
    .pipe(gulp.dest('./app/css'))
    .pipe(browserSync.reload({
      stream: true
    }))
});

gulp.task('cssnano', function() {
    return gulp.src('app/css/*.css')
      .pipe(cssnano())
      .pipe(gulp.dest('dist/css'));
});


gulp.task('pug', function () {
  return gulp.src('app/*.pug')
    .pipe(pug({
      pretty: true,
    }).on('error', notify.onError()))
    .pipe(gulp.dest('app'))
    .pipe(browserSync.reload({
      stream: true
    }))
});

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


gulp.task('watch', ['postcss', 'browserSync'], function () {
  gulp.watch('app/sass/**/*.sass', ['postcss']);
  gulp.watch(['app/*.pug', 'app/pug/**/*.pug'], ['pug']);
  gulp.watch('app/js/**/*.js', browserSync.reload);
});

gulp.task('clean', function() {
    return del.sync('dist'); 
});

gulp.task('build', ['clean', 'sass', 'scripts'], function() {

    var buildCss = gulp.src([ 
        'app/css/*.css',
        ])
    .pipe(gulp.dest('dist/css'))

    var buildFonts = gulp.src('app/fonts/**/*') 
    .pipe(gulp.dest('dist/fonts'))

    var buildJs = gulp.src('app/js/**/*') 
    .pipe(gulp.dest('dist/js'))

    var buildHtml = gulp.src('app/*.html') 
    .pipe(gulp.dest('dist'));

});

gulp.task('clear', function () {
    return cache.clearAll();
})

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

Answer 1

Solved a similar error by writing this task
.pipe(autoprefixer()) like this .pipe(postcss([ autoprefixer() ]))

Answer 2

Hello! Please help me to solve the problem:
1.

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

Gives an error message:
2.

gulp.task('img', function() {
  return gulp.src('app/img/**/*')
    .pipe(imagemin({
      Interlaced: true,
      progressiv: true,
      svgoPlugins: [{removeViewBox: false}],
      une: [pngquant()]
    }))
    .pipe( gulp.desk('dist/img') );
});

Gives an error message: