[Vue warn]: Unknown custom element: – did you register the component correctly?

You need to do the following:

import Vue from 'vue';
import VueRouter from 'vue-router';
    
Vue.use(VueRouter);

Ref: https://router.vuejs.org/en/installation.html

new Vue({
  el: "#app",
  router: router,
  render: h => h(App),
})

Hope it will help you