JS SCRIPT NOT RUNNING

I am creating a basic page in w3spaces and while I've finished editing the html and css part, I'd like to add a few scripts in it. I tried to add this script (https://www.w3schools.com/howto/howto_js_countdown.asp), which is a countdown timer to my own page but it seems like it doesn't run. Initially I used the code provided <p id="demo"></p> and nothing happened. I also tried other things such as <span id="demo" class="demo"></span> and <script src="./countdown.js"></script> but neither worked. Do you have any suggestion please as I am not very experienced.