You may not remove this last writable Windows Server 2012 domain controller because one or more read-only domain controllers ...

You may not remove this last writable Windows Server 2012 domain controller because one or more read-only domain controllers (RODCs) exist in the "%1" domain. An RODC requires a writable domain controller that runs Windows Server 2008, Windows Server 2008 R2, or Windows Server 2012 in order to function.