As per my recollection, agent will need to be installed on the VM. If the VM was cloned or deployed from a template, I think the agent has to be uninstalled and then reinstalled for it to work properly. As per the support ESXi, that is something that should be reached out to them.
As a follow on from the above answer which I agree upon if you want to use Vmware Consolidated Backup it is not available to ESXi, if you wish to do image level backups of your VM’s you will need to invest some money in vSphere essentials which is the base level for ESX now and this will allow you to use VCB. VCB requieres a sevcie console to interact with backups whcih ESXi does not have. Hope this helps