$.AJAX Javascript to PHP variable

I’ve got some problems with passing a variable through ajax from my javascript to php in the next file.

This is code I used to send the data of the room_id (if i go to console log it shows that ajax sends it well, but it keeps showing me the error

Error: Data not found in POST

What could be the problem here? Filenames and variables are okay. Thanks for help in advance.

        $.ajax({
            type: 'POST',
            url: 'functions.php',
            data: { room_id: selectedRoom }, 
            success: function(response) {
                console.log(response);
            },
            error: function(error) {
                console.error(error);
            }
        });
    });
});

and this code to recive that in PHP

 if ($_SERVER["REQUEST_METHOD"] == "POST") {
   
    if (isset($_POST['room_id'])) {
      
        $room_id = $_POST['room_id'];

        // Dodaj debug statement
        echo "Received room_id: " . $room_id;

       

        
        $response = $room_id;


        echo $response;
    } else {
       
        echo "Error: key not included 'room_id' in POST data.";
    }
} else {
   
    echo "Error: Data not found in POST.";
}

I’ve tried to get the variable from js to php to use it variable in SELECT query.