1. Open PowerShell as Administrator and give the following command to find the installed version of Server 2016:

DISM /Online /Get-CurrentEdition
2. Then give the following command to convert the Server 2016 Evaluation version to Full Retail (Licensed):

To learn version available for the current image
DISM /online /Get-TargetEditions

DISM /online /Set-Edition:ServerStandard /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX /AcceptEula