Hi I just noticed the changes between version 3 and version 3.1 In this latest version is it necessary that the secondary zone is created on a web domain? You can not create secondary zones simply assigned to the customer? Regards
I may not understand your question correctly, but DNS is completely unrelated to websites, and a customer (client) could add a Secondary DNS zone without having a website created at all.
This is not a website. You enabled the domain limit module in ISPConfig under System > Interface config so you have to add domains first under clients. If you don't want to use the domain module, then disable it again.