Thanks for the report.
I merged a change in FIX: handle email aliases when normalize emails is enabled - Pull Request #42344 - discourse/discourse - GitHub that makes the failure show a clear explanation instead of a raw validation error - and in the UI it now fails immediately rather than emailing you a confirmation link that errors when you click it.
It still doesn’t make the alias addable when normalize_emails is enabled though, as I think the uniqueness rule on the normalized address is deliberate.