Answer
The List needs to be configured as an element first, before it becomes selectable as page content — go to the All Elements section in Page Designer, select List, click the '+' button to add a new list element, and specify the Entity and other details there. Only after the List element has been created under All Elements will it appear as an available option when adding it to the page's actual content — an empty dropdown at the content-adding step means the element itself hasn't been configured yet.