I don't understand how to make the computer check if it's between two values?



No error message, I don't understand where to start. I'm not sure if this is right.

var userChoice = prompt("Do you choose rock, paper or scissors?");
var computerChoice = Math.random()

if (computerChoice <=0.33) {
    computerChoice = "rock"
} else if (computerChoice <=0.66) {
    computerChoice = "paper"
} else {
    computerChoice = "scissors"


You are doing just fine....

add a couple of console.log()'s to your code

console.log( computerChoice );

so you can see what is happening.