Tag not monitored by Microsoft.
Not sure where you got your "free serial" key but windows 10 is not free. You'll need to purchase a license or otherwise come up with a valid product key in order to activate. Windows licensing is not supported here on QnA. They're actively answering questions here in dedicated forums.
https://answers.microsoft.com/en-us/windows/forum/windows_10-win_licensing
--please don't forget to Accept as answer if the reply is helpful--
Regards, Dave Patrick ....
Microsoft Certified Professional
Microsoft MVP [Windows Server] Datacenter Management
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