How do I activate Windows 11 on a new machine from Windows 10 (on an old machine)?

[Windows Server]

I recently upgraded my old PC to a new (self assembled) machine and upgraded to Windows 11 through the "free" download. To activate, it says my old product key is not valid on this device. I was to understand my old activation was kept in my Microsoft Account, but it is asking me for a product key, which I do not have, to activate this OS.Please advise.

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