Daryl code with some fixes.

<?php

  session_start();
  require("p_db_connection.php");
  $username = $_POST['username'];
  $password = $_POST['password'];
  if($username<>'' and $password<>'')
  {
    #You shouldnt be using md5 or sha1 for passwords
    #read this https://crackstation.net/hashing-security.htm
    $encrypt_password = sha1($password);
    $sql = "SELECT * FROM useraccount WHERE user_name = '".$username."' AND user_pass = '".$encrypt_password."' AND user_status = 'Activated' LIMIT 1 ";
    $result = mysqli_query($conn, $sql);
    if(mysqli_errno($con)>0)
    {
      echo "Query error " . mysqli_errno($con);
      #dont leak information ( to hackers ) displaying mysqli_error($con)

      #handle the error

    } else {
      $count = mysqli_num_rows($result);
      if($count > 0)
      {
        while($info_result = mysqli_fetch_assoc($result))
        {
          $_SESSION['user_name'] = $username;
          if($info_result['user_privileges'] === "Admin")
          {
            header("Location: ../user_profile.php?login=success");
            exit;
          }
          if($info_result['user_privileges'] === "Reporting User")
          {
            header("Location: ../user_profile_r.php?login=success");
            exit;
          }
          if($info_result['user_privileges'] === "Monitoring User")
          {
            header("Location: ../user_profile_m.php?login=success");
            exit;
          }
          if($info_result['user_privileges'] === "External User")
          {
            header("Location: ../user_profile_e.php?login=success");
            exit;
          }
        }
      }
    }
  }
  $_SESSION['error_login'] = "Your username and password did not match in our system.";
  header("Location: ../login.php?error=didnotmatch");

?>