abhilashhb is right.
As addiontial note i can say its totally normal that you only can buy the current version and than you must downgrade if needed for no costs. If you find the action menu within your my.vmware account than its self explaining. A later upgrade it also for free and you can also perform partially downgrades or splitting instances. Dont get confused... vCenter have its own licenses.
Quick info... your vSphere 6 key than is valid from 6.0 up to 6.7u3.
If possible run the latest vSphere vCenter like 6.7 and manage your 6.0/6.5/6.7 Hosts.
If you only bought vSphere Essentials(non plus) than you have no support at first and the support end of 6.0 isnt that important for you.
Regards,
Joerg