I'm dealing with a small collection of ESXi hosts that are supposed to be in a cluster, but whomever tried to set up vCenter Server made a hash of it, and it "didn't work". So I uninstalled and reinstalled Server, and am able to connect to it just fine. When I went to add a host, vCenter Server sees and is happy to add that host, but it came up with the license key from "somewhere" (in quotes because, as a fresh install, how the heck did it know???), but then warns me "The license key you have selected is a part of another vcenter inventory. If you choose this key, it will be added to this vcenter's inventory. To ensure compliance, you will need to remove the key from its original inventory." What "original inventory"? How do I find and fix???
Since your old vCenter Server is crashed and I'm assuming you will not bring that vCenter back, just ignore the message and select the option "Use Key" if available when adding the host to your new vCenter.
if you are not using the key for any other active vcenter , no worries you can proceed by adding to new vcenter