Can you share the exact domain you're trying to add and the exact error text you see when the registration step fails? With those two details we can pinpoint what's blocking it.

One thing worth checking in the meantime: while your workspace is still on the trial, you can add one custom domain per service. If you've already added one, the next attempt will fail until you're on a paid plan. If that's not it, the specific domain and error will tell us where it's stuck.