I just installed TrueNAS and spent a good 30 minutes trying to figure out the same problem; I was incredibly surprised to find out that only root can log in the GUI.
This is a highly desirable feature for pro-users and essential one for business users. I am genuinely surprised as FreeBSD certainly offers a lot of flexibility in terms of users' management.
Does anyone have a reason for this? Is it possible to request these kinds of features? Thanks.
Yes. It should not come as a surprise that it has been requested multiple times. Unfortunately, it was never implemented in Core, though Scale has this feature.