Notice: Array to string conversion in C:\xampp\htdocs\PROJEKTBS\cart.php on line 62 [duplicate]

I got notice that says

Notice: Array to string conversion in C:\xampp\htdocs\PROJEKTBS\cart.php on line 62

Fatal error: Uncaught Error: Call to a member function fetch_assoc() on bool in C:\xampp\htdocs\PROJEKTBS\cart.php:64 Stack trace: #0 {main} thrown in C:\xampp\htdocs\PROJEKTBS\cart.php on line 64

and my sources are

<?php
            $total = 0;
            if(!empty($_SESSION['cart'])){
            $conn = new mysqli('localhost', 'root', '', 'tbodyshop');
            $index = 0;
            if(!isset($_SESSION['qty_array'])){
              $_SESSION['qty_array'] = array_fill(0, count($_SESSION['cart']), 1);
            }
            $sql = "SELECT * FROM produk WHERE id_produk IN (".implode(',',$_SESSION['cart']).")";
            $query = $conn->query($sql);
              while($row = $query->fetch_assoc()){
                ?>

line 62 is $sql