About Lesson
It turns out, that it’s NOT recommended to use async components for routing (you may use them to conditionally load and fetch component code when working with v-if etc. though).
For routing, simply change the syntax from
const CoachDetail = defineAsyncComponent(() => import('./pages/coaches/CoachDetail.vue') );
to
const CoachDetail = () => import('./pages/coaches/CoachDetail.vue');
(for all routes)