Within the Biddy Tarot Community, usernames are used a little differently from what you may be used to. Their only purpose is to provide a unique identification for users who may have the same first name within the Community.
With that in mind, the only place they are shown is on your profile, as can be seen in the picture below.
To determine how other members view you within the Community, you can set up your first name to be your display name and unselect the Display username instead of full name button, as has been done in the picture below.
If you ever have trouble saving or updating your profile, it may be due to having the same username as another member. Change your username to something more unique and try saving again.