Windows Server 2016 Troubleshooting: domain and forest level show “Technical Preview”
Issue
When you install Server 2016 then try to promote it as Domain Controller, Forest and Domain Functional Level come to “Technical Preview” but that’s not correct cause this is a Server 2016 RTM version.
Troubleshooting
When you start to install updates to Server 2016, then try to promote it as DC and it gets correct FFL & DFL.
Solution
The trick is you should fully patch Windows Server 2016 first and then promote as Domain Controller. (Also need to install following Cumulative Update.)
See also
Also, check this “Cumulative Update for Windows Server 2016: September 26, 2016”