You downloaded the Evaluation version of Windows Server 2016. When you receive your license keys you get the error “This edition cannot be upgraded” when attempting to apply them.
Fortunately a quick DISM command will resolve this allowing you to activate your server installation
Open a CMD window
Enter the following – DISM /Online /Set-Edition:ServerStandard /ProductKey:your-new-product-key-here /AcceptEula
Hit Enter and wait until its done.
Answer Y to restart your server to effect the changes.
Bug alert: After the reboot check that its activated or click on Activate and reenter the key if required
Windows Server 2016 Eval
Windows Server 2016 Standard