The reason why you hitting this is that you have enabled to allow users to register via quick registration on your login page while you did not set up to show custom fields into the quick registration form.

If you do not want to allow users to register via quick registration then you have to turn it off from the backend setting, you can refer to my attached screenshot below.

Or if you do allow them to register via quick registration then you have to set the mandatory custom field to show into the quick registration form from the backend user builder form.