If i use lifecycle on vSphere 1 & 2 it's ok
If i use lifecycle on vSphere 3 there is this message : unknown error in VMware vSphere Lifecycle Manager
I could not find the log location in the documentation as it is not updated yet I guess, however try to look into /var/log/vmware for some vsphere-lcm logs or something similar as it is called as "lcm" on all the places.
Also go to the menu of LCM and try to disable the Adminission Control check as pointed here: https://docs.vmware.com/en/VMware-vSphere/7.0/com.vmware.vsphere-lifecycle-manager.doc/GUID-472964C3...
What if you try to run the upgrade pointing to the host individually and not to the cluster? Does it work? In which phase are you seeing the Unknown Error?