Check to see if you have hosts in maintenance mode or otherwise in trouble.
Add the following permissions to the service account either at the DataCenter level or the vSphere level.
Basically, the service account doesn’t have enough vSphere account permissions, or user doesn’t have them set at the right level.
Virtual Machine / Interaction / Device Connection
Virtual Machine / Interaction / VMware Tools install
Virtual Machine / Provisioning / Clone template
Virtual Machine / Provisioning / Clone virtual machine
For required permissions refer :