What is wrong with gulpfile.js?

Question

gulp.js

Error while running gulp

TypError; (Error invalid arg type)^
The path argument must be of type string( аргумент пути должен иметь тип string). received type undefined.At assert patch. js; 39;11
at Objekt. relative

etc.

global.hostname = "localhost";

var gulp = require('gulp'),
sass = require('gulp-sass'),
autoprefixer = require('gulp-autoprefixer'),
minifycss = require('gulp-minify-css'),
rename = require('gulp-rename');

gulp.task('express', function() {
  var express = require('express');
  var app = express();
  app.use(require('connect-livereload')({port: 35729}));
  app.use(express.static(__dirname + '/app'));
  app.listen('80', hostname);
});

var tinylr;
gulp.task('livereload', function() {
  tinylr = require('tiny-lr')();
  tinylr.listen(35729);
});

function notifyLiveReload(event) {
  var fileName = require('path').relative(__dirname, event.path);
  tinylr.changed({
    body: {
      files: [fileName]
    }
  });
}

gulp.task('styles', function () {
  gulp.src('sass/*.sass')
  .pipe(sass({
    includePaths: require('node-bourbon').includePaths
  }).on('error', sass.logError))
  .pipe(rename({suffix: '.min', prefix : '_'}))
  .pipe(autoprefixer({
    browsers: ['last 15 versions'],
    cascade: false
  }))
  .pipe(minifycss())
  .pipe(gulp.dest('app'));
});

gulp.task('watch', function() {
  gulp.watch('sass/*.sass', gulp.parallel('styles'));
  gulp.watch('app/*.css', notifyLiveReload);
  gulp.watch('app/*.html', notifyLiveReload);
});

gulp.task('default', gulp.parallel('styles', 'express', 'livereload', 'watch'));

What can be done?