getElementById在脚本中未返回任何内容或问题？

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我正在编写一个程序，该程序从输入字段返回值，然后根据条件将字段字符串更改为x结果。衷心感谢您的帮助，因为这里的成员过去一直对我有很大帮助。调试器抛出此错误，当然没有任何作用：

script.js:22 Uncaught TypeError: Cannot set property 'innerHTML' of null
    at uberChecker (script.js:22)
    at HTMLButtonElement.onclick (index.html:25)

这是我的代码（这里是初学者）：

JS：

    var username = document.getElementById("myUsername").value;
    var groupIs = document.getElementById("myGroup").value;
    var warnings = document.getElementById("myWarning").value;
    var postCount = document.getElementById("myPostCount").value;





 function uberChecker() {

    if ((groupIs != ('Uber' || 'uber') && postCount > '1000') && warnings === '0') {

        document.querySelector("output-box").innerHTML = "You can become Uber !";
    } else if (postCount < '1000' && warnings === '0') {
        document.querySelector("output-box").innerHTML = (username + ' You have ' + postCount + ' posts, you do not meet the requirements');

    } else if (warnings != '0' && postCount > '1000') {
        document.querySelector("output-box").innerHTML = (username + ' You cannot upgrade with ' + warnings + ' warning')
    } else if (postCount < '1000' && warnings != '0') {
        document.querySelector("output-box").innerHTML = (username + ' you have ' + postCount + ' posts which is less than 1000 and you have ' + warnings + '% warning. You cannot upgrade');
    } else {
        document.querySelector("output-box").innerHTML = (username + ' You are already Uber');
    }

 }

HTML：

<!DOCTYPE html>

<html>

<head>
<link rel="stylesheet" type="text/css" href="style.css">

</head>

<body>

<div class="blue-box"></div>

<input type="text" placeholder="Type username" id="myUsername" class="user" >
<input type="text" placeholder="Type user group" id="myGroup" class="group">
<input type="text" placeholder="Type post count" id="myPostCount" class="post">
<input type="text" placeholder="Type warning level" id="myWarning" class="warning">

<div class="UsernameText">Username</div>
<div class="groupText">Group</div>
<div class="postText">Posts</div>
<div class="warningText">Warning</div>


<button type="button" onclick="uberChecker();" class="black-box">Check if you are upgradeable</button>

<div class="output-box">Result will display here</div>

<script src="script.js"></script>

</body>

</html>