Why do these cards not line up into a row

Issue

I'm trying to make a row of cards but the cards are staying in a column and not going into a row like my flex-direction should tell them to go.

I want it to look like this

https://css-tricks.com/snippets/css/a-guide-to-flexbox/:

But it looks like this

image.png:

body {
  font-family: 'Open Sans', sans-serif;
}

.shell {
  flex-direction: row;
  border: 1px solid #EF9A9A;
  flex-basis: auto;
  margin: 5px;
}

.card {
  width: 150px;
  display: flex;
  flex-direction: column;
  border: 1px solid #EF9A9A;
  border-radius: 4px;
  overflow: hidden;
  margin: 0px;
}

.card-header {
  color: #D32F2F;
  text-align: center;
  font-size: 12px;
  font-weight: 600;
  border-bottom: 1px solid #EF9A9A;
  background-color: #FFEBEE;
  padding: 5px 10px;
}

.card-main {
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  padding: 15px 0;
}

.material-icons {
  font-size: 36px;
  color: #D32F2F;
  margin-bottom: 5px;
}

.main-description {
  color: #D32F2F;
  font-size: 12px;
  text-align: center;
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  font-family: Arial, Helvetica, sans-serif;
}

.header {
  overflow: hidden;
  background-color: #FFEBEE;
  padding: 20px 10px;
  border-bottom: 1px solid #EF9A9A;
  border-radius: 4px;
}

.header a {
  float: left;
  color: #D32F2F;
  text-align: center;
  padding: 12px;
  text-decoration: none;
  font-size: 18px;
  line-height: 25px;
  border-radius: 4px;
}

.header a.logo {
  font-size: 25px;
  font-weight: bold;
}

.header a:hover {
  background-color: #dfd5d7;
  color: #942626;
}

.header a.active {
  background-color: #D32F2F;
  color: #FFEBEE;
}

.header-right {
  float: right;
}

@media screen and (max-width: 500px) {
  .header a {
    float: none;
    display: block;
    text-align: left;
  }
  .header-right {
    float: none;
  }
}

<link href="https://fonts.googleapis.com/icon?family=Material+Icons" rel="stylesheet">
<link href="https://fonts.googleapis.com/css?family=Open+Sans:400,600" rel="stylesheet">

<div class="header">
  <a href="#home" class="logo">Project-LuLo</a>
  <div class="header-right">
    <a class="active" href="#home">Home</a>
    <a href="#games">Games</a>
    <a href="#contact">Contact</a>
  </div>
</div>

<div class="shell">
  <div class="card">
    <div class="card-header">Games</div>
    <div class="card-main">
      <i class="material-icons">videogame_asset</i>
      <div class="main-description">Web Based Games</div>
    </div>
  </div>

  <div class="card">
    <div class="card-header">Games</div>
    <div class="card-main">
      <i class="material-icons">videogame_asset</i>
      <div class="main-description">Web Based Games</div>
    </div>
  </div>
</div>

Solution

On your class .shell you had styles to dictate what to do with display: flex; but the shell class itself also needed to have display flex on it.

body {
  font-family: 'Open Sans', sans-serif;
}

.shell{
display: flex;
flex-direction:row;
border: 1px solid #EF9A9A;  
flex-basis: auto;
margin:5px;
}

.card {
  width: 150px;                
  display: flex;              
  flex-direction: column;       
  border: 1px solid #EF9A9A;    
  border-radius: 4px;
  overflow: hidden;
  margin: 0px;              
}


.card-header {
  color: #D32F2F;
  text-align: center;
  font-size: 12px;
  font-weight: 600;
  border-bottom: 1px solid #EF9A9A;
  background-color: #FFEBEE;
  padding: 5px 10px;
}

.card-main {
  display: flex;              
  flex-direction: column;     
  justify-content: center;    
  align-items: center;        
  padding: 15px 0;            
}

.material-icons {
  font-size: 36px;
  color: #D32F2F;
  margin-bottom: 5px;
}

.main-description {
  color: #D32F2F;
  font-size: 12px;
  text-align: center;
}

* {box-sizing: border-box;}

body { 
  margin: 0;
  font-family: Arial, Helvetica, sans-serif;
}

.header {
  overflow: hidden;
  background-color: #FFEBEE;
  padding: 20px 10px;
  border-bottom: 1px solid #EF9A9A;
   border-radius: 4px;
}

.header a {
  float: left;
  color: #D32F2F;
  text-align: center;
  padding: 12px;
  text-decoration: none;
  font-size: 18px; 
  line-height: 25px;
  border-radius: 4px;
}

.header a.logo {
  font-size: 25px;
  font-weight: bold;
}

.header a:hover {
  background-color: #dfd5d7;
  color: #942626;
}

.header a.active {
  background-color: #D32F2F;
  color: #FFEBEE;
}

.header-right {
  float: right;
}

@media screen and (max-width: 500px) {
  .header a {
    float: none;
    display: block;
    text-align: left;
  }
  
  .header-right {
    float: none;
  }
}

<!DOCTYPE html>
   <head>
      <meta charset="utf-8"/>
     <title>Project-LuLo</title>
     <meta content="width=device-width, initial-scale=1" name="viewport"/>
      
   </head>
    <body>
<link href="https://fonts.googleapis.com/icon?family=Material+Icons" rel="stylesheet">
<link href="https://fonts.googleapis.com/css?family=Open+Sans:400,600" rel="stylesheet">

<div class="header">
  <a href="#home" class="logo">Project-LuLo</a>
  <div class="header-right">
    <a class="active" href="#home">Home</a>
    <a href="#games">Games</a>
    <a href="#contact">Contact</a>
  </div>
</div>

<div class="shell"> 
<div class="card">
  <div class="card-header">Games</div>
  <div class="card-main">
    <i class="material-icons">videogame_asset</i>
    <div class="main-description">Web Based Games</div>
  </div>
</div>

<div class="card">
  <div class="card-header">Games</div>
  <div class="card-main">
    <i class="material-icons">videogame_asset</i>
    <div class="main-description">Web Based Games</div>
  </div>
</div>
</div>

    </body>
</html>

Answered By - JDawwgy