How to enable button when checbox is checked

I have codes like this

<div class="checkbox">

<input type="checkbox" id="checkme" value ="accept"/>
<label>I have read and agree to the terms and conditions</label>
<p><input type="submit" name="submit" value="Order now!" id="sub1" disabled="disabled"/></p>

I was trying to put this Jscript below of codes:

<script>
$(document).ready(function() {
var the_terms = $("#checkme");

the_terms.click(function() {
if ($(this).is(":checked")) {
$("#sub1").removeAttr("disabled");
} else {
$("#sub1").attr("disabled", "disabled");
}
});
});
</script>
However it does not work at all. I already follow all guides on internet. Anyone can help what part i did wrong? Is there any additional codes beside these?'

Oh and this on php format

EDIT:
Done this too

<script>
var checker = document.getElementById('checkme');
var sendbtn = document.getElementById('sub1');
checker.onchange = function() {
sendbtn.disabled = !!this.checked;
};
</script>

But how do i change to disable when unchecked?