This error occurs when you want to include a generic Event in your method. My code looked like this:

  doReorder(ev: Event) {
    console.log('Dragged from index', ev.detail.from, 'to', ev.detail.to);
 }

The error was:

error TS2339: Property 'detail' does not exist on type 'Event'.

Solution

You can typecast to make it go away:

  doReorder(ev: Event) {
    const customEvent = ev as CustomEvent<ItemReorderEventDetail>;
    // The `from` and `to` properties contain the index of the item
    // when the drag started and ended, respectively
    console.log('Dragged from index', customEvent.detail.from, 'to', customEvent.detail.to);

    // Finish the reorder and position the item in the DOM based on
    // where the gesture ended. This method can also be called directly
    // by the reorder group
    customEvent.detail.complete();
  }