The error message is says that the source vcenter, that is the current one using 6.0 is turned off ?
Is it Powered Off ?
what does the message-button give you for information ?
If the VM is poweredOn:
What are the settings that you used ?
If you are using DRS, could the source vCenter be migrated to another host ?
I did the migration a couple of weeks ago, and somewhere in the steps you can specify which host/vcenter is managing this "old" vCenter. If you specify a host, you have to make sure that this vCenter won't migrate to another host. You can turn of DRS on the cluster or on vm-level. (its a VM Override-setting on cluster-level, but that way you can let DRS schedule all other VM's except your vCenter)
Did you specify the source vcenter by IP or DNS, and can you reach that vCenter from your workstation where the installation is running ?