flexbox doesnt show my numbers horizontally

enter image description here

Thought this would show the numbers 1,2,3,4 in the progress-container next to each other but it doesnt. What am I missing here? Must be something wrong with my understanding of flexbox..

@import url("https://fonts.googleapis.com/css?family=Muli&display=swap");

* {
  box-sizing: border-box;
}

body {
  background-color: #f6f7fb;
  font-family: "Muli", sans-serif;
  display: flex;
  align-items: center;
  justify-content: center;
  height: 100vh;
  overflow: hidden;
  margin: 0;
}

.container {
  text-align: center;
}

.progress-container {
  display: flex;
  justify-content: space-between;
  margin-bottom: 30px;
  max-width: 100%;
  width: 350px;
}


    <div class="container">
      <div class="progress-container">
        <div class="progress" id="progress">
          <div class="circle active">1</div>
          <div class="circle">2</div>
          <div class="circle">3</div>
          <div class="circle">4</div>
        </div>
      </div>

display: flex needs to be applied to the direct parent of the items that you want to be flex items. In your code, your flex container (.progress-container) has only one child (i.e. the #progress element), which doesn't have much effect (and no effect on the grandchildren).

If you move the flex settings to the #progress element, it will work as desired:

@import url("https://fonts.googleapis.com/css?family=Muli&display=swap");
* {
  box-sizing: border-box;
}

body {
  background-color: #f6f7fb;
  font-family: "Muli", sans-serif;
  display: flex;
  align-items: center;
  justify-content: center;
  height: 100vh;
  overflow: hidden;
  margin: 0;
}

.container {
  text-align: center;
}

.progress-container {
  margin-bottom: 30px;
  max-width: 100%;
  width: 350px;
}

#progress {
  display: flex;
  justify-content: space-between;
}

<div class="container">
  <div class="progress-container">
    <div class="progress" id="progress">
      <div class="circle active">1</div>
      <div class="circle">2</div>
      <div class="circle">3</div>
      <div class="circle">4</div>
    </div>
  </div>
</div>