redirect to another page and display alert message error

i created a login form inside a modal , that the user will input his username pass and his purpose, and when success it redirectly go to homepage, Now my problem is i created another page that if the user type the wrong username or password he will be redirect to another page above the form i want to display an error alert message incorrect username or password. Can you help me work this. advance thanks.
here is the sample form that will display the error message:
<form class="needs-validation "  action="#" method="post" novalidate >
  <h3 class="dark-grey-text text-center">
    <strong><i class="fa fa-user"></i> Login</strong>
  </h3>
  <hr>

  <span id="error_message" class="alert alert-danger"></span>

  <div class="row" style="margin-left:300px;">
    <label>Date:</label> 
    <span id="date" name="date"></span>
  </div>
  <div class="row" style="margin-left:300px;">
    <label>Time:</label>
    <span id="clock" name="time"></span>
  </div> 

  <div class="md-form form-sm mb-5" style="margin-top:05px;">
    <i class="fa fa-user prefix"></i>
    <input type="text" id="username3" name="username3" class="form-control form-control-sm " required>
    <label for="modalUsername">Username</label>
    <div class="invalid-feedback" style="margin-left:30px">
      Enter username.
    </div>
  </div> 
</form>
and here is my php code:
<?php
include_once ('connection.php');
if(isset($_POST['submit']))
{
    $username1 = $_POST['username1'];
    $password2= $_POST['password2'];
    $purpose =$_POST["purpose"];
    $date=date("Y-m-d");
    $time=date("H:i:s");

    $sql = "select * from visitor  where  uname = '$username1' and pass = '$password2'";
    $result = mysqli_query($conn,$sql);
    $count = mysqli_num_rows($result);

    if ($count == 0) {
      $('#error_message').html("Incorrect username or password"); 
      header("Location:login.php");
    } else {
      while ($row = mysqli_fetch_array($result)) {
        $username1 = $row['uname'];
        $password2 = $row['pass'];
        $fname=$row['fname'];
        $lname=$row['lname'];

        $InsertSql = "INSERT INTO visitor_att(fname,lname,uname,pass,purpose,date,timein) VALUES ('$lname','$fname',
        '$username1','$password2','$purpose','$date','$time')";
        $res = mysqli_query($conn, $InsertSql);
        header("Location:test.php?");
      }
  }
}
?>