I have two domains. In the first one, I want to create a subdomain using my own domain name, and it works. For example, my domain is mete.com, and I can create auth.mete.com. But I can’t do this with the other domain I createdtoday. Why?
(other information and details relevant to your question)
That’s no longer the case for new accounts, only older accounts have this. I think this was changed a year ago or so.
That’s why it worked on your original account but not the new account.
The reason for this is that sub-subdomains cannot be protected with SSL anymore. Which is of course very bad for an auth subdomain.
As an alternative, you can either setup the auth endpoint in a subdirectory (e.g. metas.gt.tc/auth) or create a separate free subdomain (e.g. metas-auth.gt.tc).