VMware introduced a significant architectural change in vSphere 7 regarding the vCenter Server architecture. If you are running vSphere 6.0, you typically cannot upgrade directly to vSphere 7 in a single step; you must migrate to 6.5 or 6.7 first, and then proceed to 7.
This blog post provides essential information regarding the current status and availability of vSphere 6 downloads. vsphere 6 download