Uncaught Error: Call to a member function bind_param() on bool

I’m building a website as a self-learning project. Now I’m making a Login/Register window. I’ve written a php file to make a login form interact with sql database but something went wrong and as I know very little of PHP I can’t fix it. Can you pls take a look?

The website is here - http://nativecheck.epizy.com/

When you click Login on the left the popup appears where you enter username and password.

Then you click enter and the data has to go to db but this error appears

> Fatal error: Uncaught Error: Call to a member function bind_param() on bool in /home/vol14_5/epizy.com/epiz_24228201/htdocs/action_page.php:21 Stack trace: #0 {main} thrown in /home/vol14_5/epizy.com/epiz_24228201/htdocs/action_page.php on line 21

My PHP code

<?php
$username = $_POST['username'];
$password = $_POST['password'];

if ( !empty($username) || !empty($password) ) {
   $host =  'server';
   $dbUsername = 'username';
   $dbPassword = 'userpassword';
   $dbname = "epiz_Logins";

   $conn = new mysqli($host, $dbUsername, $dbPassword, $dbname);

   if (mysqli_connect_error()) {
      die('Connect Error('. mysqli_connect_errno(). ')'. mysqli_connect_error());
   }
   else {
   $SELECT = 'SELECT username From register Where username = ? Limit 1';
   $INSERT = 'INSERT Into register (username, password) values (?, ?)';

   $stmt = $conn->prepare($SELECT);
   $stmt->bind_param( s , $username);
   $stmt-> execute();
   $stmt->bind_result($username);
   $stmt->store_result();
   $rnum = $stmt->num_rows;

   if ($rnum==0) {
      $stmt->close();

      $stmt = $conn->prepare($INSERT);
      $stmt->bind_param('ss', $username, $password);
      $stmt->execute();
      echo "New record added";
   }
   else {
      echo "This username has already been taken. Please choose another one";

   }
   $stmt->close();
   $conn->close();

   }
} 
else {
     echo "Fill all fields please";
     die();
}
?>