How to put an image on the background?

Question

css

I can’t put an image on the background
if I put it on a div, then it will only be on what is nested in this block
Here is what kind of background should stand up and how

And yes, I’ll add that the background is not even put in the body
Tried to find pieces of code in test pages in different ways works on my site is not.

.poster {
  width: 1420px;
  background-image: url('https://www.film.ru/sites/default/files/styles/thumb_1024x450/public/trailers_frame/mv5bmze1zdi3.jpg');
}

Answer 1

The div doesn't seem to have a height.
https://codepen.io/lord-dantes/pen/vYNZPxO