Hi @Anonymous ,
You will need to buy a Windows Server 2016 Standard product key from a official Microsoft retailer store, once you have bought the product key, you can activate your Windows Server 2016.
You can also contact the Microsoft store for more support over here:
https://support.microsoft.com/en-us/account-billing/contact-microsoft-store-support-4f615f2a-6bbd-fd69-6695-ae213d63eef0
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Best regards,
Leon