The connection cannot proceed because authentication is not enabled and the remote computer requires that authentication be enabled to connect.
This error occurs when you try to establish a remote connection from a Windows Server 2008R1 to a Windows Server 2016. In modern versions of Windows, like Windows 10 or Windows Server 2016, the RDP defaults have changed. The default for the Security Layer has changed from 0 to 2.
To resolve this issue, on the Windows Server 2016, open the registry (regedit.exe) and navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\TerminalServer\WinStations\RDP-Tcp. Then, change the SecurityLayer to 0.