Cannot use import statement outside a module (Vue js)

Issue: SyntaxError: Cannot use import statement outside a module

From import VueRouter from "vue-router" inside router.js;

router.js

import VueRouter from "vue-router";
import Vue from "vue";

. . . . .

export { router, routes };

vue.config.js

const path = require('path');
const PrerenderSpaPlugin = require('prerender-spa-plugin');
const routes = require('./src/router.js').routes

module.exports = {
  pluginOptions: {
    sitemap: {
      baseURL: 'https://example.com',
      routes
    }
  }
};