You can fix the error by checking the $_POST['score'] variable exsits, so before your query you can use
if(isset($_POST['score'])) {
$score = $_POST['score'];
} else {
$score = 0;
}
and then use the $score variable in your query, this wont fix the reason the $_POST['score'] variable isnt being sent, but its always best to check whats being sent and also in your case make sure its a number to stop people doing stuff they shouldnt be able to.
if(isset($_POST['score']) && is_numeric($_POST['score'])) {
$score = $_POST['score'];
} else {
$score = 0;
}
I'm working at the moment so cant have a look at the capx, I will try and help when finished.