Google Sites changes your entire script. You have to be a bit more careful writing JavaScript.
Adding var
in front of every variable will fix your problem:
<script>
function calcul(){
var x = parseFloat(document.getElementById("value1").value);
var y = parseFloat(document.getElementById("value2").value);
document.getElementById("answer").innerHTML=x+y;
}
</script>
<form action="" id="nothing">
<input type="text" id="value1">
<input type="text" id="value2">
<input type="button" value="Calculate" id="but" onclick="calcul()" />
</form>
<p id="answer"></p>
This will work ;-)