please have a look here: https://www.virtuallyghetto.com/nested-virtualization/nested-esxi-virtual-appliance
For vCenter you don´t need an OVA, just use the installer in the ISO-image.
Moderator: Moved to vCenter Server
When you install or upgrade vcenter you don't download an ova file first. You will need to download an iso file and start the wizard inside the iso. This wizard will automatically deploy the ova inside the iso after you've answered a few questions.
Download its appliance from the VMware site: https://my.vmware.com/group/vmware/details?downloadGroup=VC670&productId=742&rPId=22641
Then mount the ISO file, choose the installer based on your client OS type (MAC, Linux or Windows) and execute it
Deploy the appliance on the ESXi host step by step in the wizard, First Deployment phase and then configure it.Please mark my comment as the Correct Answer if this solution resolved your problem