Active Directory functional level isn't Windows Server 2003 or later [ForestLevelNotWin2003Native]
Exchange Server 2016 Setup can't continue because the Active Directory forest functional level of the target forest isn't Windows Server 2003 native or later. Before you can install Exchange 2016, you must raise the forest functional level to Windows Server 2003 or later.
For information about how to raise the forest functional level, see Raise the Forest Functional Level.
For more information about Active Directory functional levels, see the following topics:
Having problems? Ask for help in the Exchange forums. Visit the forums at: Exchange Server.