Using post template in collections

I’m trying to get a particular post to show up directly from a menu item. I have in routes.yaml these collections.

    permalink: /campaigns/dickson/{slug}/
    template: index
    filter: primary_tag:dickson
    permalink: /campaigns/{slug}/
    template: index
    filter: primary_tag:campaigns
    permalink: /{slug}/
    template: index

I tried changing the template to “post” but I got a blank page.

Firstly, why the blank page?

Secondly, is there a way to achieve this?

In the near future, this idea would be counter-productive, as I want to add other posts. What does making a post “featured” do?