There are time when you install a Windows Server using the evaluation version and decided to buy the license. Microsoft has set that the product key must match the Edition of the server. For Example, StandardEval and ServerStand is 2 different SKU.
To solves this, you will need to convert the edition using the DISM command. Make sure you run this from elevated command prompt
DISM /online /Set-Edition:ServerStandard /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX /AcceptEula