Firstly its not

document.getElementById('password');
  value;

its

document.getElementById('password').value;

Secondly, there is no action property present I’ll suggest removing the entire form tags

<div class="wrapper">
          <h3>login</h3>
        
          <div class="username">
            <input type="text" placeholder="enter username" id="username" name="usernmame" value="">
          </div>
          <div class="password">
            <input type="password" placeholder="enter password" id="password"">
          </div>
          <button type=" submit" id="btn" onclick="validate()">login</button>
          </div>
        
          <script>
            function validate() {
              let username = document.getElementById('username').value;
              let password = document.getElementById('password').value;
        
              if (username == 'please' && password == 'work') {
                alert('finally');
              } else {
                alert("NOOOO")
              }
            }
        
          </script>

on your for, you are using attibute method="post" which has alternative of method="get" which being sent using URLs you are using method="post" which has a missing attribute action="/action_page.php" that will process you’re page.
Like this

<form class="box" action="/action_page.php" method="post">

since you don’t have action attribute, and has method=”post”, the post is being sent to the same page you are sending and without receiving it properly like in php.

$username = $_POST['username'];

If you still want to continue using javascript at test it, remove at post method, and remove the type=”submit” on your button as it behaves on submitting if you just want to test using javascript.

Here is your final script.

HTML

<form class="box">
      <h3>login</h3>

      <div class="username">
        <input type="text" placeholder="enter username" id="username" name="usernmame" value="">
      </div>
      <div class="password">
        <input type="password" placeholder="enter password" id="password"">
      </div>
      <button id="btn" onclick="validate()">login</button>
    </form>
  </div>

JS

function validate(){
  let username = document.getElementById('username').value;

  let password=document.getElementById('password').value;
 

  if(username =='please' && password == 'work')
  {
    alert('finally');
  } else{
    alert("NOOOO")
  }
}

• I’m still learning, so if there’s any help, or the answer is really trivial like something I need to put before hand, an explanation of the reason why this is happening would be greatly appreciated!

This has been a problem ever since I have started using it for weekend projects. Whenever I make a button, for example one that I have been trying to use is

  <button type="submit" id="btn"  onclick="validate()">login</button>

However, when I click on the button, instead of showing me what its supposed to show, it just states this on a gray page.

This page isn’t working
If the problem continues, contact the site owner.
HTTP ERROR 405

HTML

  <div class="wrapper">
    <form class="box"  method="post">
      <h3>login</h3>

      <div class="username">
        <input type="text" placeholder="enter username" id="username" name="usernmame" value="">
      </div>
      <div class="password">
        <input type="password" placeholder="enter password" id="password"">
      </div>
      <button type="submit" id="btn"  onclick="validate()">login</button>
    </form>
  </div>

JS
//I do understand that this is not a good way of setting up a username and password ,since anyone can easily get it. Ive been just doing this as a weekend project, i just want it to show an alert if it works or not

function validate(){
  let username = document.getElementById('username');
  value;
  let password=document.getElementById('password');
  value;

  if(username =='please' && password == 'work')
  {
    alert('finally');
  } else{
    alert("NOOOO")
  }
}

I have tried to see if it was a problem with my js, but nothing seems to change, so that is why im starting to suspect that it its the button thats causin the problem