'Javascript shows me TypeError saying my variable is undefined

Im trying to code a simple Javascript exercise where I have to place different text at different part of the page but it keeps throwing me a

TypeError:sal[0] is undefined

Here is the javascript code.

function sals(a,b,c,d,e,id)
{
    var sal = document.getElementsByClassName(id);
    sal[0].style.top=a;
    sal[0].style.left=b;
    sal[0].style.color=c;
    sal[0].style.fontsize=d;
    sal[0].style.zindex=e;
}

 sals('5%','50%','yellow','250px','20','GB');

What am I doing wrong?

For further reference, here is my HTML code aswell

<html>
<head>
<title>Hello</title>
<link rel="stylesheet" href="main.css">
</head>
<body>
    <h1 class="GB"> Holla</h1>
    <script src="main.js"></script>
</body>
</html>
javascripthtml


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