It should be more clear that the trial version is limited.
I spent ages trying to follow this guide with the trial version of the standard plan. The only error message I got said that I couldn't make any more aliases. Which was confusing, because I hadn't made any yet, and I specifically chose my plan because I wanted to use a custom domain. It would have been really helpful if it had told me to go to Add Credit to your Balance, and make a payment before I could add a custom domain.