您需要导出default（exports.default = FUNCTION）并将其指向您想要的默认函数（compile/watch），或者您需要运行您的gulp，如gulp compile/gulp watch。

const gulp = require("gulp");
const sass = require("gulp-sass");
const browserSync = require("browser-sync").create();

function compile() {
  return gulp
    .src("app/scss/*.scss")
    .pipe(sass())
    .pipe(gulp.dest("app/css"))
    .pipe(browserSync.stream());
}

// Watch for changes
function watch() {
  browserSync.init({
    server: "./app/",
    index: "./index.html",
  });

  gulp.watch("app/scss/*.scss", style);
  gulp.watch("./*.html").on("change", browserSync.reload);
  gulp.watch("./js/*.js").on("change", browserSync.reload);
}

exports.compile = compile;
exports.watch = watch;
exports.default = compile;