Javascript not working as intended, code copied from W3 schools website

I’ve copied the code for a product filtering system from the W3 schools website (includes JavaScript, CSS and html) but it’s not working as intended. I’m doing this for a school project and the code is working perfectly fine for my classmates who also copied it straight from the W3 schools website. I showed my teacher my code and they couldn’t see the issue. There are 6 buttons, including a “show all” option which can be used to filter through a range of products. This is what it looks like when the products page is first opened (hasn’t been interacted with yet):

enter image description here

The “Show all” option is selected by default (shown by the grey colour)

However, when I try selecting another button, the previous button selected doesn’t revert to the colour white, instead it stays grey:

enter image description here

This eventually results in all the buttons turning grey (in this example I haven’t selected the two rightmost buttons). This makes it difficult to tell which filter is currently selected.

I know it’s a very minor detail, but I would greatly appreciate any help because my project is due very soon (:

As I said, all my code was copied directly from the W3 schools website, and I have checked many times that the code is identical, however the problem persists. I will paste and screenshot the CSS and JavaScript code because I don’t think the problem is in the HTML, but if you think the problem could be in my html code, then I can show you that as well. Another option is that the issue could be to do with my ‘over-arching’ CSS code which is used to style both the home and products pages, so I can include that if you’d like it.

This is the JavaScript code:

            filterSelection("all") // Execute the function and show all columns
            function filterSelection(c) {
              var x, i;
              x = document.getElementsByClassName("column");
              if (c == "all") c = "";
              // Add the "show" class (display:block) to the filtered elements, and remove the "show" class from the elements that are not selected
              for (i = 0; i < x.length; i++) {
                w3RemoveClass(x[i], "show");
                if (x[i].className.indexOf(c) > -1) w3AddClass(x[i], "show");
              }
            }
            
            // Show filtered elements
            function w3AddClass(element, name) {
              var i, arr1, arr2;
              arr1 = element.className.split(" ");
              arr2 = name.split(" ");
              for (i = 0; i < arr2.length; i++) {
                if (arr1.indexOf(arr2[i]) == -1) {
                  element.className += " " + arr2[i];
                }
              }
            }
            
            // Hide elements that are not selected
            function w3RemoveClass(element, name) {
              var i, arr1, arr2;
              arr1 = element.className.split(" ");
              arr2 = name.split(" ");
              for (i = 0; i < arr2.length; i++) {
                while (arr1.indexOf(arr2[i]) > -1) {
                  arr1.splice(arr1.indexOf(arr2[i]), 1);
                }
              }
              element.className = arr1.join(" ");
            }
            
            // Add active class to the current button (highlight it)
            var btnContainer = document.getElementById("myBtnContainer");
            var btns = btnContainer.getElementsByClassName("btn");
            for (var i = 0; i < btns.length; i++) {
              btns[i].addEventListener("click", function(){
                var current = document.getElementsByClassName("active");
                current[0].className = current[0].className.replace(" active", "");
                this.className += " active";
              });

}

If a screenshot is easier to read:

enter image description here

This is the CSS code (for the entire products page, not just the button filters):

products {
  
  margin-bottom: 100px;

    /* Center website */
  .main {
    max-width: 1000px;
    margin: auto;
  }

  h1 {
    font-size: 50px;
    word-break: break-all;
    align-items: center;
    text-align: center;
    justify-content: center;
  }

  .row {
    margin: 8px -16px;
  }

  /* Add padding BETWEEN each column (if you want) */
  .row,
  .row > .column {
    padding: 8px;
  }

  /* Create three equal columns that floats next to each other */
  .column {
    float: left;
    width: 33.33%;
    display: none; /* Hide columns by default */
  }

  /* Clear floats after rows */
  .row:after {
    content: "";
    display: table;
    clear: both;
  }

  /* Content */
  .content {
    background-color: white;
    padding: 10px;
  }

  /* The "show" class is added to the filtered elements */
  .show {
    display: block;
  }

  /* Style the buttons */
  .btn {
    border: none;
    outline: none;
    padding: 12px 16px;
    background-color: white;
    cursor: pointer;
  }

  /* Add a grey background color on mouse-over */
  .btn:hover {
    background-color: #ddd;
  }

  /* Add a dark background color to the active button */
  .btn.active {
    background-color: #666;
    color: white;
  }

}

Screenshot of the CSS:

enter image description here