I understand. I can buy from directadmin.com internal license but don't know can use or not with hetzner server. I have cheap licenses account from directadmin.comYou'd need to ask Hetzner if they provide licenses of DirectAdmin, if not - your only choice is a license from directadmin.com.
YouIf you're a dedicated/VPS server provider - it's legal to use internal license in the servers you provide to your customer. That type of license cannot be re-sold or provided separately though.