Cannot change Ghost subdomain name

I am on a Ghost trial at ghost.org. Looks great. But I’d like to change my Ghost subdomain and it’s giving me an error, saying “Page Not Found.” See screenshot below.

  • What’s your URL? This is the easiest way for others to help you

It’s private, so can’t show it.

  • What version of Ghost are you using? If it’s not the latest, please update Ghost first before opening your topic

It’s hosted here, so it’s the latest.

And

  • How was Ghost installed and configured?

n/a

  • What Node version, database, OS & browser are you using?

n/a

  • What errors or information do you see in the console?

  • What steps could someone else take to reproduce the issue you’re having?

In admin > Ghost(Pro) > Domain tab > Ghost.io > enter new subdomain name