Despite what the error says, this edition can in fact be upgraded, it just takes a while and requires a reboot.
Here’s how:
- Open Powershell as an administrator.
- Run the following command DISM /online /Set-Edition:ServerStandard /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX /AcceptEula (substituting your product key and edition).
- Reboot when prompted.