Call to amember function fetch_assoc() on bool inphp\result.php:8 Stacktrace: #0 student\result.php(3): require() #1{main}thrown in result.php on lin8 [duplicate]

I can't find the problem. This is the error I'm getting Warning: Undefined array key "studentID" in D:\xampp\htdocs\IRASV5\php\result.php on line 6

Fatal error: Uncaught Error: Call to a member function fetch_assoc() on bool in D:\xampp\htdocs\IRASV5\php\result.php:8 Stack trace: #0 D:\xampp\htdocs\IRASV5\student\result.php(3): require() #1 {main} thrown in D:\xampp\htdocs\IRASV5\php\result.php on line 8

<?php
require '../php/mysql.php';

$color = ["", "#1FE7C4", "#E45C17", "#06B97B", "#8CE026", "#E1CCFF", "#5BA2CC", "#0A2E82", "#957107", "#80CF18"];
// session_start();
$id = $_SESSION['studentID'];
$sql = "SELECT * FROM tbluser WHERE userID = $id";
$uInfo = $mysql->query($sql)->fetch_assoc();


$sql = "SELECT * FROM tblplo WHERE programID = '".$uInfo['programID']."'";
$totalPlo = $mysql->query($sql)->num_rows;

$sql = "SELECT * FROM tblmarks WHERE studentID = $id";
$sMarks = $mysql->query($sql);

    //course based total co marks
$cMarks = array();
$cTotal = array();
foreach($sMarks as $marks){
    $course = $marks['courseID'];
    for($i=1; $i<=10; $i++){
        if(isset($marks["q".$i."_co"]) && $marks["q".$i."_co"]!=0){
            $co = $marks["q".$i."_co"];
            if(isset($cMarks[$course][$co])){
                $cMarks[$course][$co] += $marks["q".$i."_mark"];
                $cTotal[$course][$co] += $marks["q".$i."_max"];
            }else{
                $cMarks[$course][$co] = $marks["q".$i."_mark"];
                $cTotal[$course][$co] = $marks["q".$i."_max"];
            }
        }
    }
}

$pMarks = array();
$pTotal = array();
foreach($cMarks as $c => $v){

    $sql = "SELECT * FROM tblco WHERE courseID = '$c'";
    $plos = $mysql->query($sql);
    foreach($plos as $plo){
        $pId = $plo['ploID'];
        for($i=1; $i<=10; $i++){
            if(isset($plo["co".$i]) && $plo["co".$i]==1){
                if(isset($pMakrs[$c][$pId])){
                    $pMarks[$c][$pId] += $cMarks[$c][$i];
                    $pTotal[$c][$pId] += $cTotal[$c][$i];
                }else{
                    $pMarks[$c][$pId] = $cMarks[$c][$i];
                    $pTotal[$c][$pId] = $cTotal[$c][$i];
                }
            }
        }
    }
}

?>