Error when using prepare in mysqli

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PHP

Hello is there anyone can help me ?
i'm having a hard time trying to solve this error:

Fatal error: Uncaught Error: Call to undefined method Connection::prepare()

Here's my code :

class Connection
	{

		function __construct($hostname,$username,$password,$database)
		{
			# code...
			$this->hostname = $hostname;
			$this->username = $username;
			$this->password = $password;
			$this->database = $database;
			
		}

		function setConnection(){
			$this->connection = new mysqli($this->hostname,$this->username,$this->password,$this->database);
			if(!$this->connection) die('The application is not available at the moment. Please try again later.');
		}
	}

//heres my code for login:
class Login
	{
		
		function __construct(Connection $connection)
		{
			# code...
			$this->connection = $connection;
		}

		function setLogin($username, $password){
			$this->username = $username;
			$this->password = $password;

			$login_query = $this->connection->prepare('SELECT username, password FROM admin WHERE username =? AND password =?');
			$login_query->bind_param('ss', $this->username, $this->password);
			$login_query->execute();

			$checkIfExist = $login_query->fetch();
			if($checkIfExist['name'] == $this->username && $checkIfExist['password'] == $this->password){
				header('location: hello.php');
			}

		}
	}

//and heres my code in my main index.php:
        include_once 'database.connection.php';
	include_once 'database.login.php';

	$dbconnect = new Connection('localhost','root','','inventory');
	$dbconnect->setConnection();

	$dblogin = new Login($dbconnect);

	if(isset($_POST['username']) && isset($_POST['password'])){
		$username = htmlentities(mysqli_escape_string($dbconnect->connection, $_POST['username']));
		$password = htmlentities(mysqli_escape_string($dbconnect->connection, $_POST['password']));

		$dblogin->setLogin($username, $password);
	}

Im new to class in php by the way, Thank you in advance for helping me!

What I have tried:

I Tried everything i could think of, i try to do this :

function setConnection(){
			$this->connection = new mysqli($this->hostname,$this->username,$this->password,$this->database);
			if(!$this->connection) die('The application is not available at the moment. Please try again later.');

            //i tried to return my connection using this
			return $this->connection;
		}

but no luck!