Why does this return an error? Functions, randomization, innerHTML

Question

This is my code:

function randomInt(param1){
return Math.floor(Math.random()*param1);
};
var body = document.body;
body.innerHTML += randomInt(10);

Why does this produce an error?

Answer 1

No errors, the code is working.
https://code.sololearn.com/WRD5TLkvbzzx/?ref=app

Answer 2

How come when I cut that code out and place it in the JS section I receive an error?