Js script not running

[alexxx1]

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.