Explanation/Reference:
Explanation:
Adprep just prepares the domain for Window Server 2016, it does not actually raise the domain functional level to Windows Server 2016, which is required for Device Registration.
Note: Adprep.exe is a command-line tool that is included on the installation disk of each version of Windows Server. Adprep.exe performs operations that must be completed on the domain controllers that run in an existing Active Directory environment before you can add a domain controller that runs that version of Windows Server.
Adprep.exe commands run automatically as needed as part of the AD DS installation process on servers that run Windows Server 2012 or later. The commands need to run in the following cases:
Before you add the first domain controller that runs a version of Windows Server that is later than the

latest version that is running in your existing domain.
Before you upgrade an existing domain controller to a later version of Windows Server, if that domain

controller will be the first domain controller in the domain or forest to run that version of Windows Server.
References:
https://technet.microsoft.com/en-us/library/dd464018(v=ws.10).aspx
https://technet.microsoft.com/en-us/windows-server-docs/identity/ad-fs/operations/configure-device-based- conditional-access-on-premises