I'm having trouble with my php code to mysql database . Database cannot be read

I've been writing a php code for my project but I'm having trouble reading data from my xampp database. Every time I put my username & password in the the required fields, an error comes up. I want the code to display the next form after I put in the username and the password. The database I'm using is called sales and has a table which has the fields, id, username and password. I'm new to php and if anyone out there can help me correct the code, I'll really appreciate it.

<?php
//Start session
session_start();

//Array to store validation errors
$errmsg_arr = array();

//Validation error flag
$errflag = false;

//Connect to mysql server
$link = mysqli_connect('localhost','root',"");
if(!$link) {
    die('Failed to connect to server: ' . mysqli_error());
 }

  //Select database
   $db = mysqli_select_db( $link,'sales');
   if(!$db) {
    die("Unable to select database");
  }

  //Function to sanitize values received from the form. Prevents SQL injection
  function clean($str) {
    $str = @trim($str);
    if(get_magic_quotes_gpc()) {
        $str = stripslashes($str);
    }
    return mysqli_real_escape_string($str);
  }

  //Sanitize the POST values
  $login = clean($_POST['username']);
  $password = clean($_POST['password']);

  //Input Validations
  if($login == '') {
    $errmsg_arr[] = 'Username missing';
    $errflag = true;
  }
  if($password == '') {
    $errmsg_arr[] = 'Password missing';
    $errflag = true;
  }

 //If there are input validations, redirect back to the login form
  if($errflag) {
    $_SESSION['ERRMSG_ARR'] = $errmsg_arr;
    session_write_close();
    header("location: index.php");
    exit();
  }

  //Create query
  $qry="SELECT * FROM user WHERE username='$login' AND password='$password'";
  $result=mysqli_query($qry);

  //Check whether the query was successful or not
  if($result) {
    if(mysqli_num_rows($result) > 0) {
        //Login Successful
        session_regenerate_id();
        $member = mysqli_fetch_assoc($result);
        $_SESSION['SESS_MEMBER_ID'] = $member['id'];
        $_SESSION['SESS_FIRST_NAME'] = $member['name'];
        $_SESSION['SESS_LAST_NAME'] = $member['position'];
        //$_SESSION['SESS_PRO_PIC'] = $member['profImage'];
        session_write_close();
        header("location: main/index.php");
        exit();
    }else {
        //Login failed
        header("location: index.php");
        exit();
    }
  }else {
    die("Query failed");
  }
  ?>

Afterwards,it should redirect to this form (index.php) which is located on a different folder. You an download the file here (https://drive.google.com/file/d/1vIuKOtG0v9eZmKEnJQk3LR3-0p6wqtnd/view?usp=sharing)