mysqli - PHP : Fatal error: Call to a member function fetch_assoc() on boolean

I have this code :

$sql = "SELECT * FROM trading 
        WHERE order_status <> 'DELETE'
        ORDER BY date DESC
        LIMIT 2";
$result = $conn->query($sql);

while($row = $result->fetch_assoc()) {  << LINE 96


    if ($row["type"] == 'BUY') {
---- CODE CUT HERE ----

and it gives me this error message :

Fatal error: Call to a member function fetch_assoc() on boolean in /**/**.php on line 96

why this line 96 :

while($row = $result->fetch_assoc()) {

produce that error message?