The reason for getting the error message which says "The hostname for your server is invalid." is because the "Hostname" or "Name for your server" being used for the order contains invalid characters for example: @$%!#&*+= and so on.
Kindly name sure the "Hostname" or "Name for your server" only contains letters and numbers. You can also check the steps on how to order for RDP on the portal here: https://goldenrdp.com/index.php/knowledgebase/9/How-to-order-a-Windows-VPS-RDP-on-GoldenRDP.html
You can contact our support if you need further assistance with your order.