Hi @Michael,
For the reference - you might find this thread useful, especially later posts: Sign up without email
Also, there is a video related to this question: Adalo Experiments: how to allow users sign up with username - YouTube
As for me, when implementing username sign-up, I’ve decided to fill in the email field as well (automatically, with the same value as username), to keep the user uniqueness. I guess that only email field is checked automatically for that.