Why is my nested grid-container not creating equal width columns?

so I am new to CSS grids and I've been toying around with it. I'm trying to see if I can match the layout of this mock-up in a general sense with some experimental code I've been writing: https://ibb.co/GFYqMRx

Here is my HTML:

<!DOCTYPE html>
  <head>
    <meta charset="utf-8">
    <title></title>
    <meta name="description" content="">
        <meta name="viewport" content="width=device-width, initial-scale=1">
        <link rel="stylesheet" href="normalize.css">
    <link rel="stylesheet" href="styles.css">
  </head>
  <body class="body">


    <div class="header">Header</div>
    <div class="side-navigation">Side Navigation</div>

    <main class="main">
      <div class="item search">Search</div>
      <div class="item traffic">Traffic</div>
      <div class="item daily-traffic">Daily Traffic </div>
      <div class="item mobile-users">Mobile Users</div>
      <div class="item social-stats">Social Stats</div>
      <div class="item new-members">New Members</div>
      <div class="item recent-activity">Recent Activity</div>
      <div class="item message-user">Message User</div>
      <div class="item settings">Settings</div>
    </main>


    <script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.8.0/Chart.bundle.min.js"></script> 
    <script src="app.js"></script>
  </body>
</html>

Here is my CSS:

* {
  box-sizing: border-box;
}
.item {
  border: 1px red solid;
}

.header {
  background-color: purple;
  grid-area: header;
}
.side-navigation {
  background-color: orange;
  grid-area: side-navigation;
}
.main {
  background-color: grey;
  grid-area: main;
}

.search {
  grid-area: search;
}
.traffic {
  grid-area: traffic;
}
.daily-traffic {
  grid-area: daily-traffic;
}
.mobile-users {
  grid-area: mobile-users;
}
.social-stats {
  grid-area: social-stats;
}
.new-members {
  grid-area: new-members;
}
.recent-activity {
  grid-area: recent-activity;
}
.message-user {
  grid-area: message-user;
}
.settings {
  grid-area: settings;
}
body {
  display: grid;
  grid-template-rows: 80px 80px 1fr;
  grid-template-areas: 
  "header"
  "side-navigation"
  "main";
}


@media (min-width: 769px) {

  body {
    grid-template-columns: 80px 1fr;
    grid-template-rows: 80px 1fr;
    grid-template-areas:
    "header header"
    "side-navigation main"
  }
  .main {
    display: grid;
    grid-template-columns: 1fr 1fr;
    grid-auto-rows: 150px;
  } 
}

I was first able to create the desired layout by manipulating grid lines, but I wanted to see if I could do the same thing by solely utilizing the grid-template-areas property.

For screen sizes greater than 769px, I split the nested main container into two columns, 1fr each, but I ended up getting some weird effect with a third column being installed:

Why am I unable to create equal width columns in the nested main container? Shouldn't my grid be equally split in half if I put two fr units for grid-template-columns?

Everything was working perfectly until I applied grid to the nested main container.

And why are my grid-items jumbled up like this at the bottom of the screen?

Solution

This is because you are giving grid-areas to your elements withing your main grid, without actually using the grid-area property on main. I've removed these in the snippet and the main grid displays with 2 equal columns:

* {
  box-sizing: border-box;
}

.item {
  border: 1px red solid;
}

.header {
  background-color: purple;
  grid-area: header;
}

.side-navigation {
  background-color: orange;
  grid-area: side-navigation;
}

.main {
  background-color: grey;
  grid-area: main;
}

.search {}

.traffic {}

.daily-traffic {}

.mobile-users {}

.social-stats {}

.new-members {}

.recent-activity {}

.message-user {}

.settings {}

body {
  display: grid;
  grid-template-rows: 80px 80px 1fr;
  grid-template-areas: "header" "side-navigation" "main";
}

@media (min-width: 769px) {
  body {
    grid-template-columns: 80px 1fr;
    grid-template-rows: 80px 1fr;
    grid-template-areas: "header header" "side-navigation main"
  }
  .main {
    display: grid;
    grid-template-columns: 1fr 1fr;
    grid-auto-rows: 150px;
  }
}

<body>
  <div class="header">Header</div>
  <div class="side-navigation">Side Navigation</div>

  <main class="main">
    <div class="item search">Search</div>
    <div class="item traffic">Traffic</div>
    <div class="item daily-traffic">Daily Traffic </div>
    <div class="item mobile-users">Mobile Users</div>
    <div class="item social-stats">Social Stats</div>
    <div class="item new-members">New Members</div>
    <div class="item recent-activity">Recent Activity</div>
    <div class="item message-user">Message User</div>
    <div class="item settings">Settings</div>
  </main>


  <script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.8.0/Chart.bundle.min.js"></script>
  <script src="app.js"></script>
</body>