If I am not mistaken there’s already an admin account this is why you are not allowed to create a new one under the same domain.
Or you might need to clarify if it’s a different case.
I found a workaround at the moment to create the admin account, it’s actually an account that I want to create under a different domain name (so not the domain name I created while setting up mail-in-a-box).
Workaround is to comment line 264 and 268 in management/mailconfig.py and restart mailinabox.