Почему не срабатывает команда npm start?

Файл gulpfile.js

"use strict";

var gulp = require("gulp");
var sass = require("gulp-sass");
var plumber = require("gulp-plumber");
var postcss = require("gulp-postcss");
var autoprefixer = require("autoprefixer");
var server = require("browser-sync").create();


gulp.task('style', function() {
    return gulp.src("sass/style.scss")
        .pipe(plumber())
        .pipe(sass())
        .pipe(postcss([
            autoprefixer()
        ]))
        .pipe(gulp.dest("css"))
        .pipe(server.stream())
});



gulp.task("serve", ["style"], function() {
    server.init({
        server: ".",
        notify: false,
        open: true,
        cors: true,
        ui: false
    });

    gulp.watch("sass/**/*.{scss,sass}", ["style"]);
    gulp.watch("*.html").on("change", server.reload);
});

Файл package.json

{
  "name": "site",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "dependencies": {
    "autoprefixer": "^9.6.1",
    "browser-sync": "^2.26.7",
    "gulp": "^4.0.2",
    "gulp-plumber": "^1.2.1",
    "gulp-postcss": "^8.0.0",
    "gulp-sass": "^4.0.2"
  },
  "devDependencies": {},
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC"
}