For me this is because I lacked admin perms. I read that as "if this, that; [BUT], if this, other thing". I should have read it as, "if this, that; [AND], if this, other thing".
That is to say, someone still had to do
Under "Custom domain", type your custom domain, then click Save.
This is what I saw under the UI For Code and automation → Pages
And this is what a coworker with different perms saw,
In our case, I had the role "maintain" and my coworker had the role "admin"

