ZBrush has been around for about 20 years now. The ZBrush ID system is still fairly young by comparison. Even people who have purchased in 2016/2017 might not have a ZBrush ID for any number of reasons.
If you try to log into one of the ZBrush 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 ZBrush ID system. All web pages that use the ZBrush 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 ZBrush ID will sync with our other systems, allowing you full access.