cant insert data using Ajax

Question

    (adduser.php)
    <?php
    require('./database.php');
        $username = mysqli_real_escape_string($connection,$_POST['username']);
        $fullname = mysqli_real_escape_string($connection,$_POST['fullname']);
        $contact = mysqli_real_escape_string($connection,$_POST['contact']);
        $password = mysqli_real_escape_string($connection,md5($_POST['password']));
        $queryinsert = "INSERT INTO tbl_userinfo VALUES (null, ?, ?, ?, ?)";
        $queryconnection = mysqli_stmt_init($connection);
        if(!mysqli_stmt_prepare($queryconnection, $queryinsert)){
            echo "<script type='text/javascript'>alert('SQL ADDED ERROR');</script>";
        }
        else{
            mysqli_stmt_bind_param($queryconnection, "ssss", $username, $fullname, $contact, $password);
            mysqli_stmt_execute($queryconnection);
        }
    ?>
    
    (queryuseradmin.js)   
  let username = document.forms["userform"]["username"].value;
        let fullname = document.forms["userform"]["fullname"].value;
        let contact = document.forms["userform"]["contact"].value;
        let password = document.forms["userform"]["password"].value;
    

        $.post("../controller/adduser.php",{"username": username, "fullname": fullname, "contact": contact, "password": password},function data(){             
                        document.querySelector('.bg-successmodal').style.display='flex';
                         setTimeout(function(){ document.querySelector('.bg-successmodal').style.display='none';
                         },1500); 
                         $.post("../controller/useradminquery.php",{cache: false}, function data(data){
                            $('#tableid').html(data); 
                        });     
                         $('#userformid')[0].reset();                  
                        });  

cannot figure out the error can someone analyze this codes why it does not working to insert data it is almost 2 days still figuring out