run `gulp default`

gulpfile.js

var gulp = require("gulp");
var browserSync = require("browser-sync").create();
var sass = require("gulp-sass");
var postcss = require("gulp-postcss");
var cssnano = require("cssnano");
var autoprefixer = require('autoprefixer');

gulp.task("sass", function() {
  var plugins = [
    autoprefixer(),
    cssnano()
  ];
  return gulp
    .src("./src/sass/app.scss")
    .pipe(sass())
		.pipe(postcss(plugins))
    .pipe(gulp.dest("./assets/css"))
    .pipe(browserSync.stream());
});

// Static Server + watching scss/html files
gulp.task("serve", gulp.series("sass", function() {
    browserSync.init({
      server: "./",
    //   proxy: "localhost:8080"
    });
    gulp.watch("./src/sass/app.scss", gulp.series("sass"));
    gulp.watch("./*.html").on("change", browserSync.reload);
  })
);

// Compile sass into CSS & auto-inject into browsers
gulp.task("default", gulp.series("serve"));

package.json

"devDependencies": {
    "autoprefixer": "^9.7.2",
    "browser-sync": "^2.26.7",
    "gulp": "^4.0.2",
    "gulp-autoprefixer": "^7.0.1",
    "gulp-postcss": "^8.0.0",
    "gulp-sass": "^4.0.2"
  }