There’s a couple of ways to “unauthorised” a DHCP server in Active Directory. Sometimes AD can get in a twist and says “the DHCP can’t be authorised” as it’s already been authorised. This could be potentially related to replication time but the steps below will help you check.
Via the DHCP MMC GUI -> Right click -> “Manage Authorised Servers”
Show authorised list (basically what you see in the GUI)
Netsh DHCP show server
Remove entry: (Netsh DHCP delete server ServerFQDN ServerIP address)
Netsh DHCP delete server 2003-dc1.contoso.com 10.1.192.92
Via ASDIedit MMC