ZBrush has been around for about 20 years now. The Pixologic ID system is less than 2 years old. Even people who have purchased in 2016/2017 might not have a Pixologic ID for any number of reasons.
If you try to log into one of the Pixologic ID systems (such as My Licenses) and it tells you that there is no account for your email address, all you need to do is register on the Pixologic ID system. All web pages that use the Pixlogic ID system have an option for creating such an account. When doing so, be sure that you use the same email address that your ZBrush licenses are registered under.
Once you have registered, the Pixologic ID will sync with our other systems, allowing you full access.