I recently ran in to this error upgrading my homelab vCenter from 7.0.0.10400 to 7.0.10600:

vCenter: update installation failed, vCenter Server is non-operational

Luckily, the fix was easy – all I needed to do was to delete the file “/etc/applmgmt/appliance/software_update_state.conf”

So you just need to SSH to your vCenter and execute this command:

rm /etc/applmgmt/appliance/software_update_state.conf