Uncaught TypeError: document.getElementById(…) is null

I have no idea why I am getting this error and I can’t seem to fix it.
Could someone point out my errors?

My code:

“`

$UCIROW = “quantity” .$row[‘ID’];
<input
oninput=”UpdateCartItem(<?php echo $row[‘ID’]; ?>, <?php echo $UCIROW; ?>)”
type=”number” id=”<?php echo $UCIROW; ?>”
style=”width: 8%;”
value=”<?php echo $row[‘AMOUNT’]; ?>”
>

<script type=”text/javascript”>
function UpdateCartItem(CartID, UCIROW) {
var quantity = document.getElementById(UCIROW).value;
jQuery.ajax({
url: “cart_handlerz.php”,
data: ‘CartID=’+ CartID + ‘&quantity=’+ quantity,
type: “POST”,
success:function(data){
LoadNavbarLinks()
},
error:function (){}
});
}
</script>

“`

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