Win 8.1 product key not accepted after installing Win 10.

[Windows 10]

I did run update from Windows 8.1 to Windows 10 on my HP desktop computer. Now the system says I have to activate the Windows with the product key and when I insert the old product key it gives error message. Windows was preinstalled to the computer. There is no visible way to sort this out. The update was advertised as free.

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