[Vuejs]-Issue when trying to use Nested routes in Vuejs?

You should change

const router = new VueRouter({
  routes: [
    {
      path: "/HelloWorld/",
      name: "HelloWorld",
      component: HelloWorld,
      children: [{ path: ":id", name: "User", component: User }]
    }
  ]
});

to

const router = new VueRouter({
  mode: 'history',
  routes: [
    {
      path: "/HelloWorld/",
      name: "HelloWorld",
      component: HelloWorld,
      children: [{ path: ":id", name: "User", component: User }]
    }
  ]
});

(So, add mode: history, to the router.)

More on mode: history in Vue Router v3 here