根据这篇https://blog.csdn.net/GongWei_/article/details/109986596
配置下来，发现还是不生效，折腾半天之后发现，由于我用了router，所以我没动最后一句“createApp(App).use(router).mount(’#app’)”，我的main.js是这么改的：

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

const app = createApp(App)
app.use(ElementPlus)
createApp(App).use(router).mount('#app')

片头所提文章是这样的：

import { createApp, Vue } from 'vue';
import ElementPlus from 'element-plus';
import 'element-plus/lib/theme-chalk/index.css';
import App from './App.vue';

const app = createApp(App)
app.use(ElementPlus)
app.mount('#app')

后来发现，要这么改：

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

const app = createApp(App)
app.use(ElementPlus)
app.use(router).mount('#app')

这样的话createApp(App)才指的是一个app…
改完element-plus立马生效
（初学vue，不喜勿喷）