Html – Why cant I move the style to the CSS file?

I’m trying to make an image that does a parallax when you scroll in html, and it works as long as the <style> with the details of the image is inside the .html file, but as soon as I move it to the .css file it stops working.

This is the class I’m using

<style>
    .parallax {
      background-image: url("images/background.png");
      min-height: 100px;
      background-attachment: fixed;
      background-position: center;
      background-repeat: no-repeat;
      background-size: cover;
    }
</style>

and this is how I print it

<div class="parallax"></div>

I tried moving the class inside the css file by removing the <style> tags but it stopped working even if it gave me no errors at all

.parallax {
  background-image: url("images/background.png");
  min-height: 100px;
  background-attachment: fixed;
  background-position: center;
  background-repeat: no-repeat;
  background-size: cover;
}

I know I could just leave it inside the HTML file but I want to put that inside the css.
This page is also a redirect from another page that uses the same css file, does that have to do with the fact that this is not working?

Am I missing something? I just started html and I’m lost.