原问题代码

import { createApp } from 'vue'
import App from './App.vue'
import router from './router/index'
import { DatePicker } from "ant-design-vue";

const app = createApp(App)

app.use(router)
app.use(DatePicker);

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

因为引用的vue-router没挂载到app上，且需要确保app.mount('#app')在所有app.use后。

修改后

支持链式

import { createApp } from 'vue'
import App from './App.vue'
import router from './router/index'
import { DatePicker } from "ant-design-vue";

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