Не правильная проверка.
<!DOCTYPE html>
<html>
<head>
<title>Modals practice</title>
</head>
<body>
<h2 style="text-align: center;">Open console</h2>
<script>
var enterNum = prompt("Enter number", 100);
if (isNaN(enterNum)) {
alert("Not a number")
} else {
if (enterNum % 2) {
alert("Odd")
} else {
alert("Even")
}
}
</script>
</body>
</html>