Licences for legacy XenServer 5.x/6.x are no longer available.
XenServer 5.x and 6.x have reached their end of life in 2016.
Therefore Citrix has decommissioned the legacy activation service in March 2019, as announced since February 2018.
It is highly recommended to upgrade to a supported release of Citrix Hypervisor.
Supported release provide a much enhanced feature set, along with performance improvements and security fixes.
More information can be found at citrix.com.