Thanks for clarifying about the subdomain. While it wasn't automatically generated, it is still a *.vercel.app domain which cannot be fully owned by your team to be used like a custom domain. The root domain, vercel.app, is still owned by another team.
It looks like you've added a few custom domains to your list, but they're not connected to any projects yet. Those domains must not have been associated with any other team when you added them. But you would need to own or control those domains to set DNS records and use them with your Vercel deployments.
If you can describe what you're trying to accomplish with the DNS configuration, I would be happy to help you find an alternative solution. Otherwise, I recommend buying your own custom domain so you have full control.