Domain management is very, very preview. We haven’t opened domain registrations up yet, so the domain registration related commands/API won’t work.
Creating a domain means setting it up in the system so you can manage DNS entries.
The DNS portal is designed to give to end customers. Also very preview! The idea is that you create a portal session for a specific domain, then you can give customers a one time link they can use to manage DNS entries.
This is all part of a future Stripe for Domains product we have in mind. It’s very low priority, though, I do not think we’ll work on it for at least a year. You are probably better off using DNSimple for domains at the moment.