The display: flex; property works on one of my HTML pages but not the others - css

Question

I have no idea why the display: flex; property isn't working on my HTML page.
The issue here is that it works on one of my HTML pages but not the others. I've made sure that all of them were connected to the same external CSS file. I've re-checked everything but I still can't figure out the mistake. Is possibly related to my browser or something else?
HTML
<div class="firstheading">
<h1>
Economics
</h1>
</div>
<div class="resourcelistocntainer">
<div class="resourcelist">
<h2>
Keynesian School
</h2>
<p>
Introductory Books to Keynesianism
Resources for Keynesianism
</p>
</div>
</div>
`
CSS
.firstheading {
font-size: large;
font-family: monospace;
color: white;
background-color: rgb(150, 150, 150);
border: 3px solid black;
padding: 10px;
margin: -8px;
}
.resourcelistcontainer {
display: flex;
gap: 20px;
margin-left: -7px;
margin-right: -8px;
margin-top: 30px;
}
.resourcelist {
background-color: rgb(150, 150, 150);
color: white;
font-family: monospace;
padding: 10px;
border: 3px solid black;
}
.comscip {
display: flex;
flex-direction: column;
}
body {
background-color: rgb(60,60,60);
}
a {
text-decoration: none;
color: white;
}
a:hover {
color: green;
}

Answer 1

Try this:
.firstheading {
font-size: large;
font-family: monospace;
color: white;
background-color: rgb(150, 150, 150);
border: 3px solid black;
padding: 10px;
margin: -8px;
}
.resourcelistcontainer {
display: flex;
gap: 20px;
margin-left: -7px;
margin-right: -8px;
margin-top: 30px;
}
.resourcelist {
display: flex;
background-color: rgb(150, 150, 150);
color: white;
font-family: monospace;
padding: 10px;
border: 3px solid black;
}
.comscip {
display: flex;
flex-direction: column;
}
body {
background-color: rgb(60,60,60);
}
a {
text-decoration: none;
color: white;
}
a:hover {
color: green;
}
<div class="firstheading">
<h1>
Economics
</h1>
</div>
<div class="resourcelistocntainer">
<div class="resourcelist">
<h2>
Keynesian School
</h2>
<p>
Introductory Books to Keynesianism
Resources for Keynesianism
</p>
</div>
</div>