How to edit element class with tempermonkey

I am very new to this to please take this easy with me I was trying to change the class="button disable" to class="button allow" and class="button doneOff disabled" to class="button doneOff"(seen in Picture below), I tried this in tempermonkey but it won't work.

var robuxamount = document.getElementByClassName("button disabled");

robuxamount.class="button allow";

var robuxbalance = document.getElementById("done");
robuxbalance.class="button doneOff"

setTimeout(start,0);

start();

Can anyone help me make the script please? Picture link