Locate the main.js file and the start code is:

import { createApp } from 'vue'
import App from './App.vue'
import router from './router'
import ElementUI from 'element-ui';
import 'element-ui/lib/theme-chalk/index.css';


createApp(App).use(ElementUI);

createApp(App).use(router).mount('#app')

Error message:

reason: vue3.0 no longer supports element UI, so element plus can be used to replace
NPM to install element plus

$ npm install element-plus --save

Refer to the official website of element plus and modify the main.js code to

import { createApp } from 'vue'
import App from './App.vue'
import router from './router'
import ElementPlus from 'element-plus'
import 'element-plus/dist/index.css'

const app = createApp(App)
app.use(ElementPlus)

app.use(router).mount('#app')

Error reporting solution: