Upon unauthorizing a Windows DHCP server, you may get the following error:
Most commonly, there are two reasons for this error to occur.
If you have just unauthorized a DHCP server, it may take a while for AD to replicate the change to other domain controllers. As a result, the DHCP server may still think it's authorized, whereas it is not. Either wait for, or trigger AD replication throughout your network.
Another reason for this error to occur is the presence of another DHCP server in AD that no longer exists. You can check this with the netsh dhcp command, available on any server with the DHCP server role installed:
netsh dhcp show server
Run the command from a command prompt will query AD for the current registered DHCP servers. If there's one there that you know no longer to be present, you can unauthorize it manually like so:
netsh dhcp delete server dhcp01.contoso.com 10.0.1.2
Substitude the parameters for hostname and IP accordingly.
After removing the obsolete server(s), try refreshing the DHCP console. You should now be able to either unauthorize the DHCP server. If you have unauthorized the server itself through netsh, you'll notice that the Unauthorize command has been replaced with Authorize, meaning that the server was succesfully unauthorized through the command line.
On a sidenote: you can also authorize a DHCP server through netsh:
netsh dhcp add server dhcp02.contoso.com 10.254.1.2