You have your code in a function called percentageRecovered but you're not doing anything to call it. It also has a param you don't need. So either update your js
function percentageRecovered() {
var tr = document.getElementById("totalRecovered").innerHTML;
var tc = document.getElementById("totalConfirmed").innerHTML;
var recPercentage = (tr / tc) * 100;
document.getElementById("recPerc").innerHTML = "Percentage: " + tc;
}
and your html
<script src="index.js"></script>
<script>percentageRecovered();</script>
Or just remove the function from your js so it has only the code
var tr = document.getElementById("totalRecovered").innerHTML;
var tc = document.getElementById("totalConfirmed").innerHTML;
var recPercentage = (tr / tc) * 100;
document.getElementById("recPerc").innerHTML = "Percentage: " + tc;