how can i find the product key of my windows

Check if solution provided in below link works for you:

https://learn.microsoft.com/en-gb/answers/questions/2152630/how-to-fix-my-windows-activation-key-and-error-0x8?page=0#answer-1908221

Hi,

Do you have a previously purchased, separate Windows license—either physical (25-digit key) or digital?

Or

Are you trying to activate Windows on a computer that came pre-activated with an OEM license?

Thanks.